drm/i915: Use pipe_config state to disable ilk+ pfit

No more need to guard the write with a power well check on Haswell now
that we have proper pfit state readout: We can simply only clear the
pfit if it's actually on.

This removes some duplication of knowledge between the haswell pfit
disable and pfit state readout code about.

While at it extract a little helper for this.

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