drm/nouveau/prime: Extend DMA reservation object lock

Prior to commit 019cbd4a ("drm/nouveau: Initialize GEM object before
TTM object"), the reservation object was locked across all of the buffer
object creation.

After splitting nouveau_bo_new() into separate nouveau_bo_alloc() and
nouveau_bo_init() functions, the reservation object is passed to the
latter, so the lock needs to be held across that function as well.

Fixes: 019cbd4a ("drm/nouveau: Initialize GEM object before TTM object")
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>