drm: omapdrm: dss: Pass DSS pointer to dss_get_*_clk_source()

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

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