drm/i915/debugfs: Do not return '0' if there is nothing to return

Change functions that always return '0' to be void type.

Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Reviewed-by: Maciej Patelczyk <maciej.patelczyk@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220321122759.227091-1-andi.shyti@linux.intel.com