drm/i915/dp: Factor out intel_dp_update_sink_caps()

Factor out a function updating the sink's link rate and lane count
capabilities, 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/20240220211841.448846-9-imre.deak@intel.com