drm: omapdrm: omap_crtc_flush() isn't called with modeset locked

When performing asynchronous atomic updates the modeset lock isn't taken
around the callers of omap_crtc_flush(). This isn't an issue though, as
access to the CRTC is properly serialized. Just drop the warning.

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