drm/i915/gen9: Add HDC_CHICKEN1 to HW whitelist

Required for WaAllowUMDToModifyHDCChicken1:skl,bxt

This register is added to HW whitelist to support WA required for future
enabling of pre-emptive command execution, WA implementation will be in
userspace and it cannot program this register if it is not on HW whitelist.

v2: explain purpose of changes (Chris)

Reviewed-by: Nick Hoath <nicholas.hoath@intel.com>
Signed-off-by: Arun Siluvery <arun.siluvery@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1453412634-29238-4-git-send-email-arun.siluvery@linux.intel.com
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
index 511732e..ed887cf 100644
--- a/drivers/gpu/drm/i915/i915_reg.h
+++ b/drivers/gpu/drm/i915/i915_reg.h
@@ -6045,6 +6045,8 @@
 #define  HDC_FORCE_NON_COHERENT			(1<<4)
 #define  HDC_BARRIER_PERFORMANCE_DISABLE	(1<<10)
 
+#define GEN8_HDC_CHICKEN1			_MMIO(0x7304)
+
 /* GEN9 chicken */
 #define SLICE_ECO_CHICKEN0			_MMIO(0x7308)
 #define   PIXEL_MASK_CAMMING_DISABLE		(1 << 14)