drm/i915/kbl: 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)
v4: Split out the skl part in separate commit for easier backport

References: HSD#2227156, HSD#2227050
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1465309159-30531-20-git-send-email-mika.kuoppala@intel.com

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -1227,6 +1227,9 @@ static int kbl_init_workarounds(struct intel_engine_cs *engine)
 		WA_SET_BIT_MASKED(COMMON_SLICE_CHICKEN2,
 				  GEN8_SBE_DISABLE_REPLAY_BUF_OPTIMIZATION);
 
+	/* WaDisableGafsUnitClkGating:kbl */
+	WA_SET_BIT(GEN7_UCGCTL4, GEN8_EU_GAUNIT_CLOCK_GATE_DISABLE);
+
 	/* WaDisableLSQCROPERFforOCL:kbl */
 	ret = wa_ring_whitelist_reg(engine, GEN8_L3SQCREG4);
 	if (ret)