drm/i915/execlists: Add a GEM_TRACE to show when the context is completed

Include a GEM_TRACE to show when the context is complete and we advance
the ELSP port.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180221151553.9054-1-chris@chris-wilson.co.uk