drm/omap: Call dispc timings check operation directly

Instead of call the dispc timings check function dispc_mgr_timings_ok()
from the internal encoders .check_timings() operation, expose it through
the dispc ops (after renaming it to check_timings) and call it directly
from omapdrm. This allows removal of now empty omap_dss_device
.check_timings() operations.

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>