drm/dp_mst: Allow DSC in any Synaptics last branch device

The Synaptics MST branch devices support DSC decompression on all their
output ports, provided that they are last branch devices (with their
output ports connected to the sinks). The Thinkpad 40B0 TBT dock for
instance has two such branch devices, a secondary one connected to one
of the output ports of the primary; hence the decompression needs to be
enabled in both branch devices to enable decompression for all the
sinks.

Based on the above add support for enabling decompression in last
Synaptics branch devices.

Cc: Lyude Paul <lyude@redhat.com>
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231030155843.2251023-7-imre.deak@intel.com