drm/i915: Fix FBC_FENCE_OFF

Having a 4 byte register at 0x321b seems unlikely as that's not
4 byte aligned. Since later platforms have more or less the same FBC
registers with new names, assume that FBC_FENCE_OFF is at 0x3218 just
like DPFC_FENCE_YOFF.

This feels like a simple typo in BSpec. 321Bh looks a lot like 3218h
after all.

Should still be tested on real hardware of course. But I don't have
any mobile gen4 systems.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_reg.h

@@ -1048,7 +1048,7 @@
 #define   FBC_CTL_IDLE_DEBUG	(3<<2)
 #define   FBC_CTL_CPU_FENCE	(1<<1)
 #define   FBC_CTL_PLANE(plane)	((plane)<<0)
-#define FBC_FENCE_OFF		0x0321b
+#define FBC_FENCE_OFF		0x03218 /* BSpec typo has 321Bh */
 #define FBC_TAG			0x03300
 
 #define FBC_LL_SIZE		(1536)