drm/i915: have *_debugfs_init() functions return void.

Since commit 987d65d0

 (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 debugfs_init() functions and have the functions return
void.

v2: convert intel_display_debugfs_register() stub to return void too.

Signed-off-by: Wambui Karuga <wambui.karugax@gmail.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.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/20200310133121.27913-15-wambui.karugax@gmail.com