OMAPDSS: get the dss version from core pdev

The output drivers get the omapdss hw version from the platform data for
their respective output device. This doesn't work with DT, as there's no
platform data for them.

Add a new function, omapdss_get_version(), which returns the dss version
from the core device, which will have platform data on DT also. The
function is exported so that users of omapdss can also use it.

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