drm/i915/dp: Reuse intel_dp_{max,effective}_data_rate in intel_link_compute_m_n()

Reuse intel_dp_max_data_rate() and intel_dp_effective_data_rate() in
intel_link_compute_m_n(), instead of open-coding the equivalent. Note
the kbit/sec -> kByte/sec unit change in the M/N values, but this not
reducing the precision, as the link rate value is based anyway on a less
precise 10 kbit/sec value.

Suggested-by: Jani Nikula <jani.nikula@linux.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/20231116131841.1588781-12-imre.deak@intel.com