drm/i915: Move semaphore specific ring members to struct

This will be helpful in abstracting some of the code in preparation for
gen8 semaphores.

v2: Move mbox stuff to a separate struct

v3: Rebased over VCS2 work

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com> (v1)
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>