drm/gem-vram: Un-export pin helpers

There are no external callers of the gem-vram pin helpers. Hence
unexport them.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Link: https://lore.kernel.org/r/20250526132634.531789-4-tzimmermann@suse.de