drm/i915: stop using seqcount for fence pruning

After waiting for a reservation object use reservation_object_test_signaled_rcu
to opportunistically prune the fences on the object.

This allows removal of the seqcount handling in the reservation object.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/322032/?series=64786&rev=1