drm/i915: Don't call DVO mode_set hook on DPMS changes

Calling the mode_set hook on DPMS changes doesn't seem to be necessary
for ns2501. Just drop it.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Tested-by: Thomas Richter <richter@rus.uni-stuttgart.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>