drm/vram: fix Kconfig

select isn't recursive, so we can't turn on DRM_TTM + DRM_TTM_HELPER
in config DRM_VRAM_HELPER, we have to select them on the vram users
instead.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20190904054740.20817-8-kraxel@redhat.com