drm/i915: Disable Render power gating

When RC6 along with Render power gating is enabled, GPU hang
happens due to lack of synchronization between GTI and Render
power gating.

v2: Updated commit message and WA name (Damien)

Change-Id: If1614206341eb52a21eadae8c5ebb2655029b50c
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Sagar Kamble <sagar.a.kamble@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>