drm/amdgpu: send msg to IMU for the front-door loading

This patch will make SMU send msg to IMU for the front-door loading, it is
required by some ASICs.

Signed-off-by: Yifan Zhang <yifan1.zhang@amd.com>
Signed-off-by: Xiaojian Du <Xiaojian.Du@amd.com>
Reviewed-by: Tim Huang <Tim.Huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>