drm/i915: Add vlv_dport_to_phy()

Add vlv_dport_to_phy() and fix up the return values of
vlv_dport_to_channel() and vlv_pipe_to_channel() to use
the appropriate enums.

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>