drm/i915/display: configure SDP split for DP-MST

Extend the SDP split audio config for DP-MST

Signed-off-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230822204818.109742-3-vinod.govindapillai@intel.com