drm/amdgpu/pm: notify driver unloading to PMFW for SMU v13.0.6 dGPU

Per requested, follow the same sequence as APU to send only
PPSMC_MSG_PrepareForDriverUnload to PMFW during driver unloading.

Signed-off-by: Le Ma <le.ma@amd.com>
Reviewed-by: Shiwu Zhang <shiwu.zhang@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>