drm/i915: cleanup context fini

I had this lying around from he original PPGTT series, and thought we
might try to get it in by itself.

With the introduction of context refcounting we never explicitly
ref/unref the backing object. As such, the previous fix was a bit wonky.

Aside from fixing the above, this patch also puts us in good shape for
an upcoming patch which allows a failure to occur in between
context_init and the first do_switch.

CC: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>