drm/i915: Avoid context dereference inside execlists_submission_tasklet

A lesson that has to be relearnt over and over again is that the request
does not keep a reference to the context and so we cannot freely
dereference the context from inside the execlists_submission_tasklet. In
particular, we try to do so in the new GEM_TRACE() so convert those over
to the port->context_id we keep for GEM debugging. This means the
tracing now depends on DRM_I915_GEM_DEBUG.

Fixes: bccd3b83

 ("drm/i915: Use trace_printk to provide a death rattle for GEM")
References: https://bugs.freedesktop.org/show_bug.cgi?id=104066
References: https://bugs.freedesktop.org/show_bug.cgi?id=104162
References: https://bugs.freedesktop.org/show_bug.cgi?id=104242
References: https://bugs.freedesktop.org/show_bug.cgi?id=104310
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Michel Thierry <michel.thierry@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171219220916.30882-1-chris@chris-wilson.co.uk