drm/i915/bxt: Clean up bxt_init_clock_gating

Add stepping check for A0 workarounds, and remove the associated
FIXME tags.
Split out unrelated WAs for later condition checking.

v2: Fixed format (PeterL)
v3: Corrected stepping check for WaDisableSDEUnitClockGating
    - Ignoring comment, following hardware spec instead. (ChrisH)
    Added description for TILECTL setting (JonB)

Cc: Peter Lawthers <peter.lawthers@intel.com>
Cc: Chris Harris <chris.harris@intel.com>
Cc: Jon Bloomfield <jon.bloomfield@intel.com>
Signed-off-by: Nick Hoath <nicholas.hoath@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_pm.c

@@ -116,18 +116,24 @@ static void bxt_init_clock_gating(struct drm_device *dev)
 
 	gen9_init_clock_gating(dev);
 
+	/* WaDisableSDEUnitClockGating:bxt */
+	I915_WRITE(GEN8_UCGCTL6, I915_READ(GEN8_UCGCTL6) |
+		   GEN8_SDEUNIT_CLOCK_GATE_DISABLE);
+
 	/*
 	 * FIXME:
-	 * GEN8_SDEUNIT_CLOCK_GATE_DISABLE applies on A0 only.
 	 * GEN8_HDCUNIT_CLOCK_GATE_DISABLE_HDCREQ applies on 3x6 GT SKUs only.
 	 */
-	 /* WaDisableSDEUnitClockGating:bxt */
 	I915_WRITE(GEN8_UCGCTL6, I915_READ(GEN8_UCGCTL6) |
-		   GEN8_SDEUNIT_CLOCK_GATE_DISABLE |
 		   GEN8_HDCUNIT_CLOCK_GATE_DISABLE_HDCREQ);
 
-	/* FIXME: apply on A0 only */
+	if (INTEL_REVID(dev) == BXT_REVID_A0) {
+		/*
+		 * Hardware specification requires this bit to be
+		 * set to 1 for A0
+		 */
 		I915_WRITE(TILECTL, I915_READ(TILECTL) | TILECTL_TLBPF);
+	}
 }
 
 static void i915_pineview_get_mem_freq(struct drm_device *dev)