drm: omapdrm: dss: Pass PLL pointer to dss_ctrl_pll_enable()

This will allow accessing the PLL data to get the DSS device pointer,
removing the need to access the global DSS private data.

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