drm/omap: add drm_rotation_to_tiler helper()

Add a helper function to convert DRM rotation to TILER rotation.

Also drop a error print that can never happen, as the DRM framework
makes sure the rotation is valid.

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