drm: omapdrm: dispc: Pass DISPC pointer to dispc_ops operations

This removes the need to access the global DISPC private data in those
functions (both for the current accesses and the future ones that will
be introduced when allocating the DISPC private data dynamically).

In order to allow the omapdrm side to call the dispc_ops with a DISPC
pointer, we also introduce a new function dss_get_dispc() to retrieve
the DISPC corresponding to the DSS.

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