[WHY] dcn20 enable usb-c dp ALT mode in dmcu. There is bug
when enable abm feature which cause system crash. dal team
will debug this bug later.

[HOW] disable dcn abm feature for dcn20.
Signed-off-by: hersen wu <hersenxs.wu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>