drm: Add helpers for programming hardware gamma LUTs

Provide helpers that program hardware gamma LUTs. Tha gamma ramp is
either provided by the driver or generated by the helper.

The DRM driver exports the GAMMA_LUT property with a fixed number of
entries per color component, such as 256 on 8-bit-wide components. The
entries describe the gamma ramp of each individual component. The new
helper drm_crtc_load_gamma_888() loads such gamma ramp to hardware. The
hardware uses each displayed pixel's individial components as indices
into the hardware gamma table.

For color modes with less than 8 bits per color component, the helpers
drm_crtc_load_gamma_565_from() and drm_crtc_load_gamma_555_from_888()
interpolate the provided gamma ramp to reduce it to the correct number
of entries; 5/6/5 for RGB565-like formats and 5/5/5 for XRGB1555-like
formats.

If no gamma ramp has been provided, drivers can use the new helper
drm_crtc_fill_gamma_888() to load a default gamma ramp with 256 entries
per color component. For color modes with less bits, the new helpers
drm_crtc_fill_gamma_565() and drm_crtc_fill_gamma_555() are available.
The default gamma ramp uses a gamma factor of 1.

For color modes with palette, drm_crtc_load_palette_8() load an 8-bit
palette into the hardware. If no palette has been specified,
drm_crtc_fill_palette_8() load a system-specific default palette. This
is currently only a grey-scale palette with increasing luminance, but
later patches can change this. For PCs, a VGA default palette could
be used.

v2:
- drop comment on gamma factor of 2.2 (Michel, Pekka)
- fix typos in commit description (Javier)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Acked-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://lore.kernel.org/r/20250520094203.30545-2-tzimmermann@suse.de

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