drm/vram-helper: Remove BO device from public interface

TTM is an implementation detail of the VRAM helpers and therefore
shouldn't be exposed to the callers. There's only one correct value
for the BO device anyway, which is the one stored in the DRM device.

So remove struct ttm_bo_device from the VRAM-helper interface and
use the device's VRAM manager unconditionally. The GEM initializer
function fails if the VRAM manager has not been initialized.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200106125745.13797-8-tzimmermann@suse.de