drm: omapdrm: if omap_plane_atomic_update fails, disable plane

omap_plane_atomic_update() calls dispc_ovl_setup(), which can fail (but
shouldn't). To make the code a bit more robust, make sure the plane gets
disabled if dispc_ovl_setup() fails, as otherwise we might get illegal
HW configuration leading to error interrupts.

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