drm/ast: Support DRM_FORMAT_XRGB1555 on the primary plane

drm/ast: Support DRM_FORMAT_XRGB1555 on the primary plane

Export DRM_FORMAT_XRGB1555 in the primary plane's format array and
handle the format in the color-format updates. With this, ast is now
feature complete wrt. color-format support.

Also update the comments in the gamma-LUT code to reflect the ast
manual's terminology.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patch.msgid.link/20260414070522.33943-5-tzimmermann@suse.de