drm/i915/display: remove I915_STATE_WARN_ON()

Remove the unused macro.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230512110444.1448231-3-jani.nikula@intel.com