drm/i915: Don't vblank wait on ilk-ivb after pipe enable

Like on hsw/bdw the pipe isn't actually running yet at this point.
This holds for both pch ports and the cpu edp port according to my
testing on ilk, snb and ivb.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=77297


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