drm/sti: remove use of drm_debugfs functions as return values

drm/sti: remove use of drm_debugfs functions as return values

Since commit 987d65d01356 (drm: debugfs: make
drm_debugfs_create_files() never fail), drm_debugfs_create_files() never
fails, and should return void. This change therefore removes it uses as
a return value in various functions across drm/sti.

With these changes, the affected functions have been changed to use a void
return value.

v2: convert sti_mixer_debugfs_init() and sti_compositor_debugfs_init()
to return void too. Also have sti_drm_dbg_init() to return 0 to avoid
build issues.

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/20200310133121.27913-10-wambui.karugax@gmail.com