According to Bsepc, this should be set by default, but isn't. See vo1c.4
"Render Engine Command Streamer", Section 1.1.14.3 "3D_CHICKEN3"

Bspec also says that we always need to set all mask bits.

v2: Add comment about the mask bits wtf.
Reviewed-by: NBen Widawsky <ben@bwidawsk.net>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>