drm/shmem-helper: Revert accidental non-GPL export

The referenced commit added a wrapper for drm_gem_shmem_get_pages_sgt(),
but in the process it accidentally changed the export type from GPL to
non-GPL. Switch it back to GPL.

Reported-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Fixes: ddddedaa

 ("drm/shmem-helper: Fix locking for drm_gem_shmem_get_pages_sgt()")
Signed-off-by: Asahi Lina <lina@asahilina.net>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230227-shmem-export-fix-v1-1-8880b2c25e81@asahilina.net