drm: omapdrm: dss: Pass omap_drm_private pointer to dss_mgr_ops

The dss_mgr_ops operations implemented by the omapdrm side have to look
up the omap_crtc objects from global variables as they are only passed a
channel number. In order to remove global variables in the omapdrm
driver pass the omap_drm_private pointer to the dss_mgr_ops. This
requires storing a pointer to the omap_drm_private in a global variable
on the DSS side as a temporary measure until the omapdrm and omapdss
drivers get merged.

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