drm/i915/display: throw out struct intel_load_detect_pipe

An error-valued pointer can handle all in one without the wrapper
struct.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230417153741.1074692-2-jani.nikula@intel.com