drm/nouveau: fix stale locked mutex in nouveau_gem_ioctl_pushbuf

If VM_BIND is enabled on the client the legacy submission ioctl can't be
used, however if a client tries to do so regardless it will return an
error. In this case the clients mutex remained unlocked leading to a
deadlock inside nouveau_drm_postclose or any other nouveau ioctl call.

Fixes: b88baab8 ("drm/nouveau: implement new VM_BIND uAPI")
Cc: Danilo Krummrich <dakr@redhat.com>
Cc: <stable@vger.kernel.org> # v6.6+
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240305133853.2214268-1-kherbst@redhat.com