drm/nouveau: create module debugfs root

Typically DRM drivers use the DRM debugfs root entry. However, since
Nouveau is heading towards a split into a core and a DRM driver, create
a module specific debugfs root directory.

Subsequent patches make use of this new debugfs root in order to store
GSP-RM log bufferes (optionally beyond a device driver binding).

Acked-by: Timur Tabi <ttabi@nvidia.com>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20241125142639.9126-1-dakr@kernel.org