drm/vram-helper: call the ttm manager debug function

This code was assuming there was a drm_mm here, don't do
that call the correct API.

v2: use the new exported interface.

Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200804025632.3868079-13-airlied@gmail.com