drm/i915: Kill intel_reset_dpio()

Both VLV and CHV handle the cmnreset stuff in the power well code now,
so intel_reset_dpio() is no longer needed.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>