drm: omapdrm: dss: Pass omap_dss_device pointer to dss_mgr_*() functions

The dss_mgr_*() functions take a channel argument to identify the
channel they operate on. This prevents the functions from accessing
driver data structures without resorting to global variables. In an
effort to remove global variables, pass the omap_dss_device pointer
associated with the channel instead. This will be used to look up the
omap_drm_private data structure to pass to the dss_mgr_ops.

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