ILI9488 alternative color modes

**Describe the problem you have/What new integration you would like**
I have an ILI9844, but the refresh time is quite long in 40MHz mode, and the 80MHz mode seems to crash my poorly designed PCB.

**Please describe your use case for this integration and alternatives you've tried:**
It could be beneficial to work in reduced color modes, beyond the already supported `18bit_mode` flag.

**Additional context**

Display color modes:
* Full color modes:
* 16.7M colors with dithering function (24-bit data, R: 8-bit, G: 8-bit, B: 8-bit)
* 262K colors (18-bit data, R: 6-bit, G: 6-bit, B: 6-bit)
* Reduced color modes:
* 65K colors (16-bit data, R: 5-bit, G: 6-bit, B: 5-bit)
* 8 colors (3-bit data, R: 1-bit, G: 1-bit, B: 1-bit)

#1125 and #1715 seem to request initial support for this display, but it seems to work fine already in current ESPhome :-)

Start by reviewing current ESPHome support for the ILI9488 and the existing `18bit_mode` option, then compare it with the listed 16-bit and 3-bit modes. Done means the requested reduced color modes work reliably and are documented; the issue names no files or tests, so the implementation scope needs clarification.

Written by the indexing model from the issue text.