drm/i915/dp: Factor out intel_dp_config_required_rate()

Factor out intel_dp_config_required_rate() 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/20240220211841.448846-7-imre.deak@intel.com