drm/qxl/ttm: call ttm manager debug (v2)

v2: use the new exported interface.
This code was poking inside a struct and assuming it was a drm_mm
at the start. Call the proper API.

Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200804025632.3868079-12-airlied@gmail.com