drm/amdgpu: add cp firmware backdoor loading triger

Triger CP ucode addr and data to backdoor load CP firmware.

Signed-off-by: Likun Gao <Likun.Gao@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>