The Synaptics MST hubs expose some sink EDID modes with a reduced HBLANK
period, presumedly to save BW, which the hub expands before forwarding
the stream to the sink. In particular a 4k mode with a standard CVT
HBLANK period is exposed with either a CVT reduced blank RBv1,v2 (80,
160 pixel) or a non-CVT 56 pixel HBLANK period. The DP standard
describes the above HBLANK expansion functionality, but it requires
enabling this explicitly, whereas these hubs apply the expansion
transparently.

In some TBT docks with such a Synaptics hub (DELL WD22TB4) the above modes
will work okay until DSC decompression is enabled in the hub for the given
sink, but after this the same mode will not work reliably in decompressed
mode. In another TBT dock (Thinkpad 40B0) the above modes will not work
in uncompressed/18bpp mode (regardless of whether DSC decompression was
enabled before or not).

As a workaround force enable DSC for such modes.  Apply the WA when the
HBLANK period is 300ns or below, matching the above tested modes with a
533.25MHz dotclock and maximum 160 HBLANK pixels.

OTOH DSC for these modes will only work above a certain compressed bpp
threshold which depends on the link rate, so apply this limit as well
in the workaround.

On platforms, pipe/port configurations where DSC is not supported, for
instance on ICL where DSC/MST is still work-in-progress, limit the
minimum link bpp to 24.

Apply the workaround only for Synaptics hubs which support the HBLANK
expansion.

v2:
- Apply the WA whenever the HBLANK period is 300ns or below.
v3:
- Clarify in the commit log the failure modes of the different docks.
- Handle platforms/pipe/port configurations without DSC support.

Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com> (v1)
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231107001505.3370108-4-imre.deak@intel.com