drm/mgag200: Enable atomic gamma lut update

Add support for atomic update of gamma lut.
With this patch the "Night light" feature of gnome3
is working properly on mgag200.

v2:
 - Add a default linear gamma function
 - renamed functions with mgag200 prefix
 - use format's 4cc code instead of bit depth
 - use better interpolation for 16bits gamma
 - remove legacy function mga_crtc_load_lut()
 - can't remove the call to drm_mode_crtc_set_gamma_size()
    because it doesn't work with userspace.
 - other small refactors

v3:
 - change mgag200_crtc_set_gamma*() argument
    to struct drm_format_info *format
 - fix printk format to %p4cc for 4cc and %zu for size_t
 - rebased to drm-misc-next.

Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>
Tested-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220513084900.1832381-1-jfalempe@redhat.com