drm: omapdrm: dss: Pass DSS private structure to runtime PM functions

To prepare for the removal of the global variable storing DSS private
data, pass its pointer to the dss_runtime_{get,put}() functions.

As this requires getting hold of the dss_device structure in the
callers, we add a new dss_get_device() function to retrieve it. The
function currently returns a pointer to the global data structure, and
will later be updated to get the pointer from device driver data when
the DSS private structure will be allocated dynamically.

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