drm/i915/kbl: Add WaDisableGamClockGating

drm/i915/kbl: Add WaDisableGamClockGating

According to bspec we need to disable gam unit clock gating on
on kbl revids A0 and B0.

References: HSD#2226858, HSD#1944358
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1465309159-30531-14-git-send-email-mika.kuoppala@intel.com
(cherry picked from commit 8aeb7f624fbf8a68a9c67f831d4158a0f80ea920)
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>