drm/amd/display: guard otg disable w/a for test

[why & how]
HW removed this w/a, but we will still keep it to avoid regression.
but return in test mode.

Reviewed-by: Charlene Liu <charlene.liu@amd.com>
Signed-off-by: Muhammad Ahmed <ahmed.ahmed@amd.com>
Signed-off-by: Tom Chung <chiahsuan.chung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>