Storing context reference into request struct
allows us to inspect context and its associated
objects when requests are retired.

Both ppgtt and arb robustness work will need
this.
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>