drm/amdgpu/pm: implement the SMU_MSG_EnableGfxImu function

GC v11_0_1 asic needs to issue the EnableGfxImu message after start IMU.

Signed-off-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Tim Huang <Tim.Huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>