drm/amd/display: Fix takover from VGA mode

drm/amd/display: Fix takover from VGA mode

BugLink: https://bugs.launchpad.net/bugs/1796786
HW Engineer's Notes:
 During switch from vga->extended, if we set the VGA_TEST_ENABLE and then
 hit the VGA_TEST_RENDER_START, then the DCHUBP timing gets updated correctly.
 Then vBIOS will have it poll for the VGA_TEST_RENDER_DONE and unset
 VGA_TEST_ENABLE, to leave it in the same state as before.

Signed-off-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@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>
(backported from commit bd9bc355be45dd2295ca746aa05b058be4cf94cc)
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>