drm/shmem-helper: Fix obj->filp derefence

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

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/20200615151026.2339113-1-daniel.vetter@ffwll.ch