drm/i915: Avoid use-after-free of ctx in request tracepoints

trace_i915_gem_request_out may be used after the request is completed,
and so the request may have been retired on another thread, invalidating
the rq->ctx. Avoid dereferencing rq->ctx in the tracepoint by switching
to the fence context id instead, updating all tracepoints to match.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170316204235.27786-1-chris@chris-wilson.co.uk
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>