drm/i915: Move DP port disable to post_disable for pch platforms

We need to turn the DP port off after the pipe, otherwise the pipe won't
turn off properly on certain pch platforms at least (happens on my ILK for
example).  This also matches the BSpec modeset sequence better. We still
don't match the spec exactly though (eg. audio disable should happen
much earlier), but at last this eliminates the nasty
wait_for_pipe_off() timeouts.

We already did the port disable after the pipe for VLV/CHV and for CPU
eDP.

For g4x leave the port disable where it is since that matches the
modeset sequence in the documentation and I don't have a suitable
machine to test if the other order would work.

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>