OMAPDSS: refactor dss probe function

Refactor dss probe function by extracting the setup for video plls into
a separate function. The call to this function is also moved to a
slightly earlier phase, so that in error case we can bail out more
easily.

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