drm/dp: extract drm_dp_dpcd_clear_payload()

SST with 128b/132b channel coding needs this too. Extract to a separate
helper, independent of MST.

Cc: Lyude Paul <lyude@redhat.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/904fa73ea9ea976185062eeb493a08ffc43ed27e.1733238941.git.jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>