drm/i915: Correct the bit number for the MI_FLUSH_ENABLE.

Older specs claimed this was bit 11, but newer specs and the actual
simulator code say it was bit 12.  Regardless, we don't use MI_FLUSH,
or try to enable it any more.
Signed-off-by: NEric Anholt <eric@anholt.net>
Reviewed-by: NKenneth Graunke <kenneth@whitecape.org>
Reviewed-by: NBen Widawsky <ben@bwidawsk.net>
[danvet: Anyone trying to use this bit, please read all the relevant
discussions, it's epic.]
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_reg.h

@@ -391,7 +391,7 @@
 
 #define MI_MODE		0x0209c
 # define VS_TIMER_DISPATCH				(1 << 6)
-# define MI_FLUSH_ENABLE				(1 << 11)
+# define MI_FLUSH_ENABLE				(1 << 12)
 
 #define GFX_MODE	0x02520
 #define GFX_MODE_GEN7	0x0229c