drm/i915/dp: Account for tunnel BW limit in intel_dp_max_link_data_rate()

Take any link BW limitation into account in
intel_dp_max_link_data_rate(). Such a limitation can be due to multiple
displays on (Thunderbolt) links with DP tunnels sharing the link BW.

Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240220211841.448846-15-imre.deak@intel.com