drm/dp: Add DSC virtual DPCD quirk for Realtek MST branch device

drm/dp: Add DSC virtual DPCD quirk for Realtek MST branch device

The ASUS DC301 USB-C dock containing a Realtek MST branch device
supports the DSC decompression functionality on each of the dock's
downstream connectors, even though there is no discoverable peer-to-peer
virtual device in the MST topology (which the DP Standard
requires/suggests to control the DSC functionality on a per-DFP basis).
Add the DP_DPCD_QUIRK_DSC_WITHOUT_VIRTUAL_DPCD quirk for this branch
device as well to enable the DSC decompression functionality on all DFP
connectors of the dock, similarly to how this is done for dock's
containing older Synaptics branch devices.

Cc: Lyude Paul <lyude@redhat.com>
Reported-and-tested-by: Shawn C Lee <shawn.c.lee@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patch.msgid.link/20260525125516.2794636-1-imre.deak@intel.com