drm: omapdrm: Don't get/put dispc in omap_crtc_flush()

The omap_crtc_flush() function is always called with a reference to the
dispc held. Remove unnecessary calls to dispc_runtime_get() and
dispc_runtime_put().

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