drm/i915/icl: restore WaEnableFloatBlendOptimization

Enables blend optimization for floating point RTs

This restores the workaround that was reverted in c358514b
("Revert "drm/i915/icl: WaEnableFloatBlendOptimization"").

The revert was due to the register write seemingly not sticking,
but the HW team has confirmed that this is because the
register is WO and that the workaround is indeed required.

Here the wa is added with a mask of 0 since the register is WO.

References: https://hsdes.intel.com/resource/1408134172
References: https://bugs.freedesktop.org/show_bug.cgi?id=107338
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Signed-off-by: Talha Nassar <talha.nassar@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/1548983324-15344-4-git-send-email-talha.nassar@intel.com

drivers/gpu/drm/i915/i915_reg.h

@@ -2801,6 +2801,9 @@ enum i915_power_well_id {
 #define GEN6_RCS_PWR_FSM _MMIO(0x22ac)
 #define GEN9_RCS_FE_FSM2 _MMIO(0x22a4)
 
+#define GEN10_CACHE_MODE_SS			_MMIO(0xe420)
+#define   FLOAT_BLEND_OPTIMIZATION_ENABLE	(1 << 4)
+
 /* Fuse readout registers for GT */
 #define HSW_PAVP_FUSE1			_MMIO(0x911C)
 #define   HSW_F1_EU_DIS_SHIFT		16

drivers/gpu/drm/i915/intel_workarounds.c

@@ -549,6 +549,12 @@ static void icl_ctx_workarounds_init(struct intel_engine_cs *engine)
 	if (IS_ICL_REVID(i915, ICL_REVID_A0, ICL_REVID_A0))
 		WA_SET_BIT_MASKED(GEN11_COMMON_SLICE_CHICKEN3,
 				  GEN11_BLEND_EMB_FIX_DISABLE_IN_RCC);
+
+	/* WaEnableFloatBlendOptimization:icl */
+	wa_write_masked_or(wal,
+			   GEN10_CACHE_MODE_SS,
+			   0, /* write-only, so skip validation */
+			   _MASKED_BIT_ENABLE(FLOAT_BLEND_OPTIMIZATION_ENABLE));
 }
 
 void intel_engine_init_ctx_wa(struct intel_engine_cs *engine)