drm/amd/display: Check aux channel before MST resume

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: Jerry (Fangzhi) Zuo <Jerry.Zuo@amd.com>
Reviewed-by: Roman Li <Roman.Li@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>