drm/i915/dp_mst: Allow modes requiring compression for DSC passthrough

drm/i915/dp_mst: Allow modes requiring compression for DSC passthrough

The DSC passthrough mode allows a compressed stream to be forwarded
to the sink instead of being decompressed at the last MST branch
device, provided that the branch device supports passthrough and
the sink supports decompression. This enables modes that would not
be possible without compression, as the bandwidth required for the
uncompressed stream exceeds what is available on the full MST path
from source to sink.

Currently, MST mode validation assumes the stream is uncompressed
and uses the corresponding uncompressed minimum link BPP for
bandwidth calculation. Use the minimum compressed link BPP instead
when DSC passthrough is available to enable the modes described
above.

The non-passthrough DSC mode, where the last MST branch device
decompresses the stream, may also allow enabling additional modes.
This would require determining the link bandwidth between the last
branch device and the sink based on the
DFP_Link_Available_Payload_Bandwidth_Number reported by the branch
device for the sink via the ENUM_PATH_RESOURCES MST message.
Supporting this is left for a follow-up for the following reasons:

1. DFP Link Available PBN reporting is not supported by any of the
   available MST devices used for testing.
2. Non-passthrough mode would enable additional modes only if the link
   bandwidth between the last branch device and the sink exceeded that
   of the full MST path. Unless multiple MST devices are used, or link
   training forces a reduced bandwidth between the source and the first
   branch device, both rare cases, this is unlikely.

Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/4332
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patch.msgid.link/20260216070421.714884-5-imre.deak@intel.com