drm/amdgpu: fix ring0 failed on pro card

the root cause is vram content is lost completely after pci reset.

Signed-off-by: Chunming Zhou <David1.Zhou@amd.com>
Reviewed-by: Roger.He <Hongbo.He@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>