drm/nouveau: Provide nouveau_bo_{pin,unpin}_locked()

Implement pinning without locking in nouveau_bo_pin_locked(). Keep
nouveau_bo_pin() for acquiring the buffer object's reservation lock.
The new helper will be useful for implementing the GEM pin callback
with correct semantics. Same for unpin.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Tested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com> # virtio-gpu
Acked-by: Christian König <christian.koenig@amd.com>
Acked-by: Zack Rusin <zack.rusin@broadcom.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240227113853.8464-6-tzimmermann@suse.de