drm/amd/display: add DP2.0 debug option to set MST_EN for SST stream

[why]
Some DP2.0 RX requires us to set MST_EN even for SST configuration.
We added this debug option so we can configure this temporary workaround
for the RX.

Reviewed-by: George Shen <george.shen@amd.com>
Acked-by: Agustin Gutierrez Sanchez <agustin.gutierrez@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>