drm/nouveau: Don't take struct_mutex around the pushbuf IOCTL.

We don't need it and it can lead to lock order inversions with respect
to drm_global_mutex, potentially causing dead locks.

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