drm/nouveau: Avoid lock dependency between ramht and ramin spinlocks.

The ramht code called some gpuobj functions with the HARDIRQ-safe
RAMHT spinlock held, this could potentially lead to a dead lock
because ramin_lock is HARDIRQ-unsafe.

Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>