drm/i915/gem: Update context name on closing

Update the context.name on closing so that the persistent requests are
clear in debug prints.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191111114323.5833-3-chris@chris-wilson.co.uk