drm/i915/ringbuffer: Implement advance using set_tail

As noted by Zhenyu, we can now simply replace the existing advance hook
by calling the new set_tail function pointer directly.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

diff --cc drivers/gpu/drm/i915/intel_ringbuffer.c
Simple merge

diff --cc drivers/gpu/drm/i915/intel_ringbuffer.h
Simple merge