drm: convert .debugfs_init() hook to return void.

As a result of commit 987d65d01356 (drm: debugfs: make
drm_debugfs_create_files() never fail) and changes to various debugfs
functions in drm/core and across various drivers, there is no need for
the drm_driver.debugfs_init() hook to have a return value. Therefore,
declare it as void.

This also includes refactoring all users of the .debugfs_init() hook to
return void across the subsystem.

v2: include changes to the hook and drivers that use it in one patch to
prevent driver breakage and enable individual successful compilation of
this change.

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/20200310133121.27913-18-wambui.karugax@gmail.com
diff --git a/include/drm/drm_gem_vram_helper.h b/include/drm/drm_gem_vram_helper.h
index 0f6e472..b63bcd1 100644
--- a/include/drm/drm_gem_vram_helper.h
+++ b/include/drm/drm_gem_vram_helper.h
@@ -196,7 +196,7 @@ static inline struct drm_vram_mm *drm_vram_mm_of_bdev(
 	return container_of(bdev, struct drm_vram_mm, bdev);
 }
 
-int drm_vram_mm_debugfs_init(struct drm_minor *minor);
+void drm_vram_mm_debugfs_init(struct drm_minor *minor);
 
 /*
  * Helpers for integration with struct drm_device