drm/i915: Remove completed fences after a wait

If we wait upon the full (i.e. all shared fences, or upon an exclusive
fence) reservation object successfully, we know that all fences beneath
it have been signaled, so long as no new fences were added whilst we
slept. If the reservation_object remains the same, as detected by its
seqcount, we can then reap all the fences upon completion.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170217151304.16665-6-chris@chris-wilson.co.uk