drm/ttm: Initialize debugfs from ttm_global_init()

We create a bunch of debugfs entries as a side-effect of
ttm_global_init() and then never clean them up.  This isn't usually a
problem because we free the whole debugfs directory on module unload.
However, if the global reference count ever goes to zero and then
ttm_global_init() is called again, we'll re-create those debugfs entries
and debugfs will complain in dmesg that we're creating entries that
already exist.  This patch fixes this problem by changing the lifetime
of the whole TTM debugfs directory to match that of the TTM global
state.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210721152358.2893314-6-jason@jlekstrand.net