drm/i915: Keep GMCH DPLL VGA mode always disabled

We disable the DPLL VGA mode when enabling the DPLL, but we enaable it
again when disabling the DPLL. Having VGA mode enabled even in unused
DPLLs can cause problems for CHV, so it seems wiser to always keep it
disabled. And let's just do that on all GMCH platforms to keep things
as similar as possible between them.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Sivakumar Thulasimani <sivakumar.thulasimani@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>