drm/fb-helper: Clip damage area horizontally

Clip the damage area horizontally if only a single scanline has been
changed. This is helpful to reduce the memcpy overhead for small writes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220209161617.3553-6-tzimmermann@suse.de