drm/gem-vram: Do not set pin and unpin callbacks

Gem-vram helpers do not support PRIME dma-buf sharing. So nothing
will ever call pin/unpin on its buffer objects. Do not set these
callbacks in struct drm_gem_object_funcs.

v2:
- fix typo in commit description

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Link: https://lore.kernel.org/r/20250526132634.531789-3-tzimmermann@suse.de