drm/debugfs: remove checks for return value of drm_debugfs functions.

Since 987d65d0

 (drm: debugfs: make drm_debugfs_create_files() never
fail), there is no need to check the return value of
drm_debugfs_create_files(). Therefore, remove remove unnecessary checks
and error handling statement blocks for its return value.

These changes also enable changing drm_debugfs_create_files() to return
void.

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/20200310133121.27913-17-wambui.karugax@gmail.com