drm/sysfb: vesadrm: Add gamma correction

Add palette support and export GAMMA properties via sysfs. User-space
compositors can use this interface for programming gamma ramps or night
mode.

Vesadrm supports palette updates via VGA DAC registers or VESA palette
calls. Up to 256 palette entries are available. Userspace always supplies
gamma ramps of 256 entries. If the native color format does not match
this because pixel component have less then 8 bits, vesadrm interpolates
among the palette entries.

The code uses CamelCase style in a few places to match the VESA manuals.

v3:
- fix coding style
v2:
- use CONFIG_X86_32 instead of __i386__ (checkpatch)
- protect struct vesadrm.pmi with CONFIG_X86_32

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://lore.kernel.org/r/20250401094056.32904-19-tzimmermann@suse.de