drm/i915: Allow some privileged commands from master

The Intel DDX uses these to implement scanline waits in the X server.

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>