drm/i915/display: split out load detect to a separate file

Load detect is shared between tv and crt but otherwise isolated in
intel_display.c.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230417153741.1074692-1-jani.nikula@intel.com