drm/omap: Factor out common mode validation code

The encoder .atomic_check() and connector .mode_valid() operations both
walk through the dss devices in the pipeline to validate the mode.
Factor out the common code in a new omap_drm_connector_mode_fixup()
function.

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>