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: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
[danvet: Anyone trying to use this bit, please read all the relevant
discussions, it's epic.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>