drm/i915/gen9: simplify DC toggling code

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Patrik Jakobsson <patrik.jakobsson@linux.intel.com>
[fix line over 80 chars checkpatch WARN in gen9_set_dc_state() (imre)]
Link: http://patchwork.freedesktop.org/patch/msgid/1446657859-9598-8-git-send-email-imre.deak@intel.com
diff --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
index 8e13f41..89018ba 100644
--- a/drivers/gpu/drm/i915/i915_reg.h
+++ b/drivers/gpu/drm/i915/i915_reg.h
@@ -7495,6 +7495,7 @@
 
 /* GEN9 DC */
 #define DC_STATE_EN			0x45504
+#define  DC_STATE_DISABLE		0
 #define  DC_STATE_EN_UPTO_DC5		(1<<0)
 #define  DC_STATE_EN_DC9		(1<<3)
 #define  DC_STATE_EN_UPTO_DC6		(2<<0)