drm: omapdrm: dispc: Pass DISPC pointer to remaining dispc API functions

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).

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