drm/i915/execlists: Emit i915_trace_request_out for preemption

Move the tracepoint into the common execlists_context_schedule_out() and
call it from preemption completion as well. A small bit of refactoring
code should help with when tracing, or else we end up with requests
mysteriously disappearing and some being emitted to HW multiple times.

Reported-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180502230202.6848-1-chris@chris-wilson.co.uk