drm/i915: Disable/enable planes as the first/last thing during modeset on gmch platforms

We already moved the plane disable/enable to happen as the first/last
thing on every other platforms. Follow suit with gmch platforms.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
[danvet: Frob drm_vblank_on conflict, as usual.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>