drm/i915/utils: throw out unused stuff

Remove some of the unused helpers from i915_utils.h.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220607094207.536699-3-jani.nikula@intel.com