drm/vboxvideo: Use managed VRAM-helper initialization

updated to use drmm_vram_helper_init().

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1614735587-6621-1-git-send-email-tiantao6@hisilicon.com