drm/i915: Enable/disable IPS when primary is enabled/disabled

IPS should be OK as long as one plane is enabled on the pipe, but
it does seem to cause problems when going between primary only and
sprite only.

This needs more investigations, but for now just disable IPS whenever
the primary plane is disabled.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>