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

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

BugLink: https://bugs.launchpad.net/bugs/1796786
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>
(cherry picked from commit abca24007e0838ee8bfff37a188bf8df00703c52)
Signed-off-by: Hui Wang <hui.wang@canonical.com>
Acked-by: Kleber Souza <kleber.souza@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>