drm/amd/display: Fix takover from VGA mode

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>