drm/i915: Remove the MI_FLUSH_ENABLE setting.

We have always been using the wrong bit -- it's bit 12.  However, the
bit also doesn't do anything -- hardware has always accepted the
MI_FLUSH command even when it was specced not to.

Given that there is only one MI_FLUSH emitted in all of the driver
stack on gen6+ (in i965_video.c of the 2d driver, and it should be
using other code to do its flush instead), just remove the MI_FLUSH
enable instead of trying to fix it.

Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>