drm/panthor: Fix build warning when DEBUG_FS is disabled

drm/panthor: Fix build warning when DEBUG_FS is disabled

Commit a3707f53eb3f ("drm/panthor: show device-wide list of DRM GEM
objects over DebugFS") causes a build warning and linking error when
built without support for DebugFS, because of a non-inline non-static
function declaration in a header file.

On top of that, the function is only being used inside a single
compilation unit, so there is no point in exposing it as a global
symbol.

This is a follow-up from Arnd Bergmann's first fix.
Also move panthor_gem_debugfs_set_usage_flags() into panthor_gem.c and
declare it static.

Fixes: a3707f53eb3f ("drm/panthor: show device-wide list of DRM GEM objects over DebugFS")
Reported-by: Arnd Bergmann <arnd@arndb.de>
Closes: https://lore.kernel.org/dri-devel/20250424142419.47b9d457@collabora.com/T/#t
Signed-off-by: Adrián Larumbe <adrian.larumbe@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Link: https://lore.kernel.org/r/20250424184041.356191-1-adrian.larumbe@collabora.com
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>