drm/amdgpu: fix mode2 reset sequence for vangogh

We need to save and restore PCI config space.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Huang Rui <ray.huang@amd.com>