drm/mgag200: Add FB_DAMAGE_CLIPS support

The driver does support damage clips, but doesn't advertise it.
So when running gnome/wayland on Matrox hardware, the full frame is
copied to the slow Matrox memory, which leads to very poor performances.

Add drm_plane_enable_fb_damage_clips() to advertise this capability to
userspace.

With this patch, gnome/wayland becomes usable on Matrox GPU.

Suggested-by: Jonas Ådahl <jadahl@gmail.com>
Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220504134026.283417-2-jfalempe@redhat.com