drm/i915: Use intel_edp_panel_vdd_on() in intel_dp_probe_mst()

We want to use the higher level vdd on func here. Not a big deal
yet (we'd just get the warn when things go awry) but when the
locking gets fixed this becomes more important.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>