drm/ast: Add plane atomic_check() functions

Introducing atomic_check() for priamry and cursor plane. The functions
validate the plane state and will later set format information for the
CRTC's atomic_flush().

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191202111557.15176-5-tzimmermann@suse.de