drm/i915: change VLV IOSF sideband accessors to not return error code

We never check the return values, and there's not much we could do on
errors anyway. Just simplify the signatures. No functional changes.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>