drm/i915/bxt: fix panel fitter setup in crtc disable/enable

Broxton has the same panel fitter registers as Skylake.

v2:
- add MISSING_CASE for future platforms (daniel)

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Sagar Kamble <sagar.a.kamble@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>