drm/i915/chv: Drop WaGsvBringDownFreqInRc6

Drop WaGsvBringDownFreq on CHV.
When in RC6 requesting the min freq should be fine to bring the
voltage down.

Signed-off-by: Deepak S <deepak.s@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@Virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>