drm/i915: Fix GT_MODE default value

I can't even find how I figured this might be needed anymore. But sure
enough, the value I'm reading back on platforms doesn't match what the
docs recommends.

It seemed to fix Chris' GT1 in limited testing as well.
Tested-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@vger.kernel.org
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_reg.h

@@ -527,6 +527,9 @@
 # define VS_TIMER_DISPATCH				(1 << 6)
 # define MI_FLUSH_ENABLE				(1 << 12)
 
+#define GEN6_GT_MODE	0x20d0
+#define   GEN6_GT_MODE_HI	(1 << 9)
+
 #define GFX_MODE	0x02520
 #define GFX_MODE_GEN7	0x0229c
 #define RING_MODE_GEN7(ring)	((ring)->mmio_base+0x29c)

drivers/gpu/drm/i915/intel_pm.c

@@ -3474,6 +3474,11 @@ static void gen6_init_clock_gating(struct drm_device *dev)
 			   DISPPLANE_TRICKLE_FEED_DISABLE);
 		intel_flush_display_plane(dev_priv, pipe);
 	}
+
+	/* The default value should be 0x200 according to docs, but the two
+	 * platforms I checked have a 0 for this. (Maybe BIOS overrides?) */
+	I915_WRITE(GEN6_GT_MODE, _MASKED_BIT_DISABLE(0xffff));
+	I915_WRITE(GEN6_GT_MODE, _MASKED_BIT_ENABLE(GEN6_GT_MODE_HI));
 }
 
 static void gen7_setup_fixed_func_scheduler(struct drm_i915_private *dev_priv)