drm/dp_mst: Mention max_payloads in proposed_vcpis/payloads docs

Mention that the size of these two structs is determined by
max_payloads. Suggested by Ville Syrjälä.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200122194321.14953-2-lyude@redhat.com