drm/i915: implement w/a for incorrect guarband clipping

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: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>