drm/simpledrm: Iterate over damage clips

Iterate over all damage clips and updated them one by one. Replaces
the merging of damage areas, which can result in significant overhead
if damage areas are not close to each other.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220922130944.27138-5-tzimmermann@suse.de