drm/i915: Make next_seqno debugs entry to use i915_gem_set_seqno

This debugs entry can be used to set arbitrary value to next_seqno.
Use i915_gem_set_seqno instead of poking next_seqno.

v2: nasty details of next_seqno and last_seqno handling
moved inside i915_gem_set_seqno as suggested by Chris Wilson.

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>