drm/mgag200: Optimize damage clips

When there are multiple damage clips, previous code merged them into one
big rectangle. As the Matrox memory is very slow, it's faster to copy each
damage clip.

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