drm/amdgpu: Fix discovery initialization failure during pci rescan

Waiting for system ready to fix the discovery initialization
failure issue. This failure usually occurs when dGPU is removed
and then rescanned via command line.
It's caused by following two errors:
[1] vram size is 0
[2] wrong binary signature

Signed-off-by: Ma Jun <Jun.Ma2@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>