drm/ast: Remove traces of DRM_FORMAT_RGB888 handling

drm/ast: Remove traces of DRM_FORMAT_RGB888 handling

There's some incomplete handling for DRM_FORMAT_RGB888 in the mode-
setting code. It was added by ast's original commit, which imported
the code from the user-space Xorg driver. But the hardware doesn't
support 24-bit pixel sizes. Hence remove the traces of RGB888 from
ast.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patch.msgid.link/20260414070522.33943-3-tzimmermann@suse.de