drm/amd/display: early return if not in vga mode in disable_vga

The work around for hw bug causes S3 resume failure. Don't execute
disable vga logic if not in vga mode.

Signed-off-by: Eric Yang <Eric.Yang2@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>