Vega10/gfx900 (Instinct MI25): PSP handshake timeout - 3-line mdelay fix, card dead on stock 7.2.x

# Title: linux-cachyos: Vega10/gfx900 (Instinct MI25) PSP handshake timeout — 3-line fix, card unusable on stock kernel

## Summary
Instinct MI25 (Vega10/gfx900) fails amdgpu init on linux-cachyos 7.2.x (and other current kernels):
PSP times out loading its secure OS, init aborts with -22, and the card is dead. A 3-line patch
(`mdelay(20)` -> `mdelay(500)` in psp_v3_1.c) fixes it completely. Verified on 7.2.4.

## Symptom (stock kernel)
```
amdgpu 0000:06:00.0: [drm] PSP loading UVD firmware
amdgpu 0000:06:00.0: psp reg (0x16080) wait timed out, mask: 8000ffff, read: 80080124 exp: 80000000
amdgpu 0000:06:00.0: PSP load sos failed!
amdgpu 0000:06:00.0: hw_init of IP block failed -22
amdgpu 0000:06:00.0: Fatal error during GPU init
amdgpu 0000:06:00.0: probe with driver amdgpu failed with error -22
```
(Failure path also NULL-derefs in drm_sched_entity_fini during teardown on some runs.)

## Root cause
psp_v3_1.c waits only 20 ms after writing PSP_BL__LOAD_SYSDRV / LOAD_SOSDRV before polling the
C2PMSG handshake. On this card (and other Vega10s, per community reports since 2022) the PSP
firmware needs longer to respond; the poll then times out and init aborts. Same failure and fix
documented 4 years ago: https://superuser.com/questions/1747738 (kernel patch mdelay 20->500ms).

## Fix (verified)
```
drivers/gpu/drm/amd/amdgpu/psp_v3_1.c: 3 hunks, mdelay(20) -> mdelay(500)
psp_v3_1_bootloader_load_sysdrv (C2PMSG_35 handshake)
psp_v3_1_bootloader_load_sos (C2PMSG_81 sign-of-life)
one more handshake site at ~line 266
```
One-time 500 ms boot delays; no hot-path cost. Patch file: https://gist.github.com/jamesonBradfield/70269fa7c4c40641699d22aaa041c854 (0001-amdgpu-psp-v3-1-handshake-delay.patch, 3 hunks, a/b format)

## Verification
- linux-cachyos 7.2.4-2 built with the patch (AUR PKGBUILD + patch): MI25 initializes cleanly
("Initialized amdgpu 3.64.0 for 0000:06:00.0"), 16GB VRAM live, full day of dual-GPU
llama.cpp Vulkan benchmarking stable, temps/fan normal.
- Reproduced the failure on both 7.2.2 stock and 6.18.48-lts stock; fixed on 7.2.4-2 patched.

## Ask
Carry this 3-line quirk patch in the cachyos kernel patchset (kernel-patches misc/), or accept an
upstream submission. gfx900 is EOL upstream so mainline likely won't take it — CachyOS carrying it
is the practical path for MI25/Vega10 users on rolling kernels. Happy to open a PR against
CachyOS/kernel-patches with the patch + this report.

## Related (separate)
Also required for this card: kernel param `pci=realloc` for the 16GB BAR ("can't assign; no
space" on the card's PCIe-switch subtree) — BIOS/firmware interaction, likely not kernel-package
territory; noting for completeness.

Start with drivers/gpu/drm/amd/amdgpu/psp_v3_1.c and inspect the three handshake sites named in the issue. Apply the supplied patch through the CachyOS kernel-patches workflow, build a kernel, and verify that the MI25 initializes without the PSP timeout and reports its VRAM successfully.

Written by the indexing model from the issue text.