drm/omap: dss: Remove omap_dss_driver .[gs]et_mirror operations

The .get_mirror() and .set_mirror() omap_dss_driver operations are
implemented by the panel-tpo-td043mtea1 driver but are never used.
Remove them.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>