drm/i915: Move reserve_seqno() next to unreserve_seqno()

Move the companion functions next to each other.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170223074422.4125-4-chris@chris-wilson.co.uk