drm: serialize access to list of debugfs files

Nouveau, when configured with debugfs, creates debugfs files for every
channel, so structure holding list of files needs to be protected from
simultaneous changes by multiple threads.

Without this patch it's possible to hit kernel oops in
drm_debugfs_remove_files just by running a couple of xterms with
looped glxinfo.

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>