drm/shmem-helper: Make all exported symbols GPL

Make all drm-shmem exported symbols GPL to make them consistent with
the rest of drm-shmem symbols.

Acked-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.d>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250322212608.40511-5-dmitry.osipenko@collabora.com