drm/shmem-helper: Only dma-buf imports are private obj

I broke that in my refactoring:

commit 7d2cd72a


Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Fri May 29 16:05:42 2020 +0200

    drm/shmem-helpers: Simplify dma-buf importing

I'm not entirely sure of the history here, but I suspect that in one
of the rebases or when applying the patch I moved the hunk from
drm_gem_shmem_prime_import_sg_table(), where it should be, to
drm_gem_shmem_create_with_handle(), which is totally wrong.

Remedy this.

Thanks for Thomas for the crucial hint in debugging this.

Tested-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Reported-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: 7d2cd72a

 ("drm/shmem-helpers: Simplify dma-buf importing")
Cc: Boris Brezillon <boris.brezillon@collabora.com>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Rob Herring <robh@kernel.org>
Cc: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200616114723.2363268-1-daniel.vetter@ffwll.ch