drm/omap: Add a dss device operation flag for .get_modes()

Instead of manually iterating over the dss devices in the pipeline to
find the first one that implements the .get_modes() operation, add a new
operation flag for .get_modes() and use the omap_connector_find_device()
helper function to locate the right dss device.

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