CRASH disassembling regdeps ISA with non-DR syntax requested

I hit this in PR #6941 where the syntax got set to DR_DISASM_ATT and then the view test crashed on its regdeps case. This crash will happen anytime regdeps is disassembled with a syntax other than DR_DISASM_DR:

```
Program received signal SIGSEGV, Segmentation fault.
0x00005555556cebbe in get_next_instr_info (info=0x0) at core/ir/x86/decode.c:2779
2779 return (const instr_info_t *)(info->code);
(gdb) bt
#0 0x00005555556cebbe in get_next_instr_info (info=0x0) at core/ir/x86/decode.c:2779
#1 0x00005555556cecf7 in get_encoding_info (instr=0x7fffffffc3b0) at core/ir/encode_shared.c:106
#2 0x00005555556de721 in instr_disassemble_opnds_noimplicit (buf=0x7fffffffc520 " 00010011 00060906 move [8byte] ", bufsz=196, sofar=0x7fffffffc4a8,
dcontext=0xffffffffffffffff, instr=0x7fffffffc3b0) at core/ir/disassemble_shared.c:1006
#3 0x00005555556dfbb4 in internal_instr_disassemble (buf=0x7fffffffc520 " 00010011 00060906 move [8byte] ", bufsz=196, sofar=0x7fffffffc4a8,
dcontext=0xffffffffffffffff, instr=0x7fffffffc3b0) at core/ir/disassemble_shared.c:1300
#4 0x00005555556de278 in internal_disassemble (buf=0x7fffffffc520 " 00010011 00060906 move [8byte] ", bufsz=196, sofar=0x7fffffffc4a8, dcontext=0xffffffffffffffff,
pc=0x7fffffffca68 "\021", orig_pc=0x7f6fdd3ec360 , with_pc=false, with_bytes=true,
extra_bytes_prefix=0x55555573adf4 "") at core/ir/disassemble_shared.c:862
#5 0x00005555556de684 in disassemble_to_buffer (drcontext=0xffffffffffffffff, pc=0x7fffffffca68 "\021",
orig_pc=0x7f6fdd3ec360 , show_pc=false, show_bytes=true,
buf=0x7fffffffc520 " 00010011 00060906 move [8byte] ", bufsz=196, printed=0x0) at core/ir/disassemble_shared.c:980
```