It is to fix: MST display failed to resume from S3

At the beginning of resume from S3, need to check if mgr->aux is
NULL. Fake MST encoder doesn't have real aux channel.
Signed-off-by: NJerry (Fangzhi) Zuo <Jerry.Zuo@amd.com>
Reviewed-by: NRoman Li <Roman.Li@amd.com>
Acked-by: NHarry Wentland <harry.wentland@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>