drm/i915/skl: Add WaDisableGafsUnitClkGating

We need to disable clock gating in this unit to work around
hardware issue causing possible corruption/hang.

v2: name the bit (Ville)
v3: leave the fix enabled for 2227050 and set correct bit (Matthew)

References: HSD#2227156, HSD#2227050
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1465309159-30531-2-git-send-email-mika.kuoppala@intel.com

drivers/gpu/drm/i915/i915_reg.h

@@ -6948,6 +6948,7 @@ enum skl_disp_power_wells {
 
 #define GEN7_UCGCTL4				_MMIO(0x940c)
 #define  GEN7_L3BANK2X_CLOCK_GATE_DISABLE	(1<<25)
+#define  GEN8_EU_GAUNIT_CLOCK_GATE_DISABLE	(1<<14)
 
 #define GEN6_RCGCTL1				_MMIO(0x9410)
 #define GEN6_RCGCTL2				_MMIO(0x9414)

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -1118,6 +1118,9 @@ static int skl_init_workarounds(struct intel_engine_cs *engine)
 			GEN7_HALF_SLICE_CHICKEN1,
 			GEN7_SBE_SS_CACHE_DISPATCH_PORT_SHARING_DISABLE);
 
+	/* WaDisableGafsUnitClkGating:skl */
+	WA_SET_BIT(GEN7_UCGCTL4, GEN8_EU_GAUNIT_CLOCK_GATE_DISABLE);
+
 	/* WaDisableLSQCROPERFforOCL:skl */
 	ret = wa_ring_whitelist_reg(engine, GEN8_L3SQCREG4);
 	if (ret)