drm/i915: Move DPIO port init earlier

To implement DPIO lane power gating on CHV we're going to need to access
DPIO registers from the cmn power well enable hook. That gets called
rather early, so we need to move the DPIO port IOSF sideband port
assignment earlier as well.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Deepak S <deepak.s@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>