drm/i915/lspcon: change signature of lspcon_detect_hdr_capability()

Switch from struct intel_lspcon to struct intel_digital_port to unify,
and return the value so the caller doesn't have to look at
lspcon->hdr_detected directly.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/6f249f5c537a16a6762faddffd6a95cbf61cf077.1738677489.git.jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>