drm/i915: Use pipe config state to control gmch pfit enable/disable

Allows us to rip out a few fragile checks (which are duplicated in the
hw state readout now, too). Also prepares us a bit for more than one
panel/pfit.

Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>