drm/i915: Rename i915_gem_timeline.next_seqno to .seqno

Rename i915_gem_timeline member 'next_seqno' into 'seqno' as
the variable is pre-increment. We've already had two bugs due
to the confusing name, second is fixed as follow-up patch.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20161124144750.2610-1-chris@chris-wilson.co.uk