drm/ttm: add debugfs directory v2

As far as I can tell the buffer_count was never used by an
userspace application.

The number of BOs in the system is far better suited in
debugfs than sysfs and we now should be able to add other
information here as well.

v2: add that additionally to sysfs

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/414954/