drm/i915/dp: Add intel_dp_max_link_data_rate()

Add intel_dp_max_link_data_rate() to get the link BW vs. the sink DPRX
BW used by a follow-up patch enabling the DP tunnel BW allocation mode.
The link BW can be below the DPRX BW due to a BW limitation on a link
shared by multiple sinks.

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/20240220211841.448846-11-imre.deak@intel.com