drm/i915/dp: Export intel_dp_max_common_rate/lane_count()

Export intel_dp_max_common_rate() and intel_dp_max_lane_count() used by
a follow-up patch enabling the DP tunnel BW allocation mode.

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/20240220211841.448846-8-imre.deak@intel.com