drm/amdgpu: Notify sbios device ready before send request

it is required if a platform supports PCIe root complex
core voltage reduction. After receiving this notification,
SBIOS can apply default PCIe root complex power policy.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org