drm/amd/display: Add work around for tunneled MST.

[Why]
Certain USB4 docks do not seem to be able to handle disabling
DSC once it has been enabled on an MST stream. This can result
in blank displays.

[How]
As a work around, always enable DSC on docks exhibiting this issue. The
flag to indicate the use of DSC for MST streams on a USB4 dock is set
during detection of the dock and only cleared when the USB4 dock is
disconnected.

Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Jimmy Kizito <Jimmy.Kizito@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>