drm/i915: implement WaDisableVLVClockGating_VBIIssue on VLV

This allows us to get the right vblank interrupt frequency.

v2: pull in register definition
Signed-off-by: NJesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: NAntti Koskipää <antti.koskipaa@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_reg.h

@@ -556,6 +556,8 @@
 #define IIR		0x020a4
 #define IMR		0x020a8
 #define ISR		0x020ac
+#define VLV_GUNIT_CLOCK_GATE	0x182060
+#define   GCFG_DIS		(1<<8)
 #define VLV_IIR_RW	0x182084
 #define VLV_IER		0x1820a0
 #define VLV_IIR		0x1820a4

drivers/gpu/drm/i915/intel_pm.c

@@ -3755,6 +3755,13 @@ static void valleyview_init_clock_gating(struct drm_device *dev)
 		   PIPEA_HLINE_INT_EN | PIPEA_VBLANK_INT_EN |
 		   SPRITEB_FLIPDONE_INT_EN | SPRITEA_FLIPDONE_INT_EN |
 		   PLANEA_FLIPDONE_INT_EN);
+
+	/*
+	 * WaDisableVLVClockGating_VBIIssue
+	 * Disable clock gating on th GCFG unit to prevent a delay
+	 * in the reporting of vblank events.
+	 */
+	I915_WRITE(VLV_GUNIT_CLOCK_GATE, GCFG_DIS);
 }
 
 static void g4x_init_clock_gating(struct drm_device *dev)