drm/nv50-/vm: take mutex rather than irqsave spinlock

These operations can take quite some time, and we really don't want to
have to hold a spinlock for too long.

Now that the lock ordering for vm and the gr/nv84 hw bug workaround has
been reversed, it's possible to use a mutex here.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>