drm/ast: Use helpers for programming gamma ramps and palettes

drm/ast: Use helpers for programming gamma ramps and palettes

Replace ast's code for programming the hardware gamma/palette LUT
with DRM helpers. Either load provided data or program a default.
Set the individual entries with a callback.

Each gamma/palette value is given as 3 individual 16-bit values
for red, green and blue. The driver reduces them to 8 bit to make
them fit into hardware registers.

v3:
- fix tags (Javier)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://lore.kernel.org/r/20250520094203.30545-3-tzimmermann@suse.de