drm/omap: drm_atomic_get_plane_state() may return ERR_PTR

drm_atomic_get_plane_state() may return ERR_PTR. Handle
drm_atomic_get_plane_state() return values right in
omap_crtc_atomic_set_property().

Signed-off-by: Jyri Sarha <jsarha@ti.com>
Acked-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>