drm/dp_mst: Use DP_MST_LOGICAL_PORT_0 instead of magic number

Use the macro defined for the first logical port instead of the
corresponding magic number.

Cc: Lyude Paul <lyude@redhat.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210201120145.350258-4-imre.deak@intel.com