drm/nouveau/imem/gk20a: Turn instmem lock into mutex

The gk20a implementation of instance memory uses vmap()/vunmap() to map
memory regions into the kernel's virtual address space. These functions
may sleep, so protecting them by a spin lock is not safe. This triggers
a warning if the DEBUG_ATOMIC_SLEEP Kconfig option is enabled. Fix this
by using a mutex instead.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Tested-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>