drm/i915: don't save/restore panel fitter registers

AFAICT i9xx_pfit_disable() on the GMCH display crtc disable path in
i9xx_crtc_disable() will always disable the panel fitter by writing 0 to
PFIT_CONTROL. The register save will always save/restore 0. Also we
completely recompue both in intel_gmch_panel_fitting so there's no way
we depend upon leftover bits.

Move the PFIT_CONTROL and PFIT_PGM_RATIOS save/restore to UMS
code. While at it, save/restore them both under the same conditions.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
[danvet: Make it a bit clearer that we nowhere depend upon these
bits.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>