drm: omapdrm: dss: Store DSS device pointer in the omapdrm private data

The dss_device is the top-level component in the omapdss driver. Give
the omapdrm driver access to the dss_device pointer in order to obtain
pointers to all other components from it. This requires a new global
variable in the omapdss driver that will be removed when merging the
omapdrm and omapdss drivers, but will already allow removal of several
other global variables.

As this partly duplicates the omapdss_is_initialized() API, reimplement
it as an inline function wrapping omapdss_get_dss().

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