drm/ast: Add cursor plane

The cursor plane uses an internal format of ARGB4444. To userspace, we
announce ARGB8888 and do the transformation internally.

v2:
	* don't set plane call-back functions to NULL explicitly
	* define plane format array in global scope

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191107083404.6852-9-tzimmermann@suse.de