drm/vram: Unconditonally set BO call-back functions

The statement's condition is always true.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190911110910.30698-5-tzimmermann@suse.de