drm/i915/tc: make intel_tc_port_get_lane_mask() static

This function is only used locally, so make it static and remove the
definition from the header file.

Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230825081638.275795-3-luciano.coelho@intel.com