drm/i915: Kill vblank waits after pipe enable on gmch platforms

The pipe might not start to actually run until the port has been enabled
(depends on the platform and port type). So don't try to wait for vblank
after we enabled the pipe but haven't yet enabled the port.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
References: https://bugs.freedesktop.org/show_bug.cgi?id=77297
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>