drm/panfrost: Make sure MMU context lifetime is not bound to panfrost_priv

Jobs can be in-flight when the file descriptor is closed (either because
the process did not terminate properly, or because it didn't wait for
all GPU jobs to be finished), and apparently panfrost_job_close() does
not cancel already running jobs. Let's refcount the MMU context object
so it's lifetime is no longer bound to the FD lifetime and running jobs
can finish properly without generating spurious page faults.

Reported-by: Icecream95 <ixn@keemail.me>
Fixes: 7282f764 ("drm/panfrost: Implement per FD address spaces")
Cc: <stable@vger.kernel.org>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>