drm/i915: fix 945 fence register writes for fence 8 and above.

The last 8 fence registers sit at a different offset, so when we went to set
fence number 8 in the lower offset, we instead set PGETBL_CTL, and the GPU
got all sorts of angry at us.

fd.o bug #20567.  Easily reproducible by running glxgears and killing it about
6 times.

Signed-off-by: Eric Anholt <eric@anholt.net>
diff --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
index 9d6539a..90600d8 100644
--- a/drivers/gpu/drm/i915/i915_reg.h
+++ b/drivers/gpu/drm/i915/i915_reg.h
@@ -184,6 +184,7 @@
  * Fence registers
  */
 #define FENCE_REG_830_0			0x2000
+#define FENCE_REG_945_8			0x3000
 #define   I830_FENCE_START_MASK		0x07f80000
 #define   I830_FENCE_TILING_Y_SHIFT	12
 #define   I830_FENCE_SIZE_BITS(size)	((ffs((size) >> 19) - 1) << 8)