drm/i915: Introduce ring set_seqno

In preparation for setting per ring initial seqno values
add ring::set_seqno().

Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

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

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