drm: make various debugfs_init() functions return 0

drm: make various debugfs_init() functions return 0

Since commit 987d65d01356 (drm: debugfs: make
drm_debugfs_create_files() never fail), drm_debugfs_create_files() never
fails and should return void. Therefore, remove its use as the
return value of various debugfs_init() functions in drm, and have these
functions return 0 directly.

v2: convert debugfs_init() functions to return 0 instead of void to
avoid build breakage.

References: https://lists.freedesktop.org/archives/dri-devel/2020-February/257183.html
Signed-off-by: Wambui Karuga <wambui.karugax@gmail.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200310133121.27913-16-wambui.karugax@gmail.com