drm/i915: Reject commands that would store to global HWS page

PIPE_CONTROL and MI_FLUSH_DW have bits that would write to the
hardware status page. The driver stores request tracking info
there, so don't let userspace overwrite it.

v2: trailing comma fix, rebased

Signed-off-by: Brad Volkin <bradley.d.volkin@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>