drm/i915: Enable DVO between mode_set and dpms hooks

To more closely match the IEGD ns2501 driver behaviour, call the
mode_set hook while the DVO port is still disabled, then enable the DVO
port, and finally call the dpms hook.

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>