drm/i915/tc: move legacy code out of the main _max_lane_count() func

This makes the code a bit more symmetric and readable, especially when
we start adding more display version-specific alternatives.

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/20230825081638.275795-4-luciano.coelho@intel.com