OMAPDSS: move 'dss_initialized' to dss driver

We have a flag, 'dss_initialized', which tells omapfb and omapdrm if
omapdss is available. At the moment it can be set even if the dss
submodules are not all ready, in case something gets deferred.

Move the flag to dss_core driver so that it'll signal the availability
of the dss drivers move accurately.

For now, it'll signal that dss_core is ready, which is not quite correct
but still better than previously.  The following patches will add
component system to omapdss, and after those patches 'dss_initialized'
will signal that all the submodules are ready.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>