drm/amd/display: Refine disable VGA

drm/amd/display: Refine disable VGA

BugLink: https://bugs.launchpad.net/bugs/1796786
bad case won't follow normal sense, it will not enable vga1 as usual, but vga2,3,4 is on.

Signed-off-by: Clark Zheng <clark.zheng@amd.com>
Reviewed-by: Tony Cheng <tony.cheng@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 219be9dda6137bc9759c449bbff5d4394fe73382)
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>