drm/omap: move dss_initialized to omapdss-base

omapdss_is_initialized() is used to find out if omapdss has been probed
successfully. This patch moves the related code to the common
omapdss-base module, so that the same support will be there for both
omapdss and omapdss6.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>