drm/i915: Fix tracing of submit seqno

We pre-increment the timeline->seqno when handing it to the request,
make sure the GEM_TRACE takes this into account. Otherwise, it appears
that we go backwards over a preemption point:

1d..1 157681077us : __i915_request_unsubmit: vcs0 fence 75e:3 <- global_seqno 17
0d.s1 157681113us : __i915_request_submit: vcs0 fence 75e:3 -> global_seqno 16

Fixes: d9b13c4d ("drm/i915: Trace GEM steps between submit and wedging")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180322110059.4467-1-chris@chris-wilson.co.uk