drm/amd/pm: skip PrepareMp1ForUnload message in s0ix

The documentation around PrepareMp1ForUnload message says that
anything sent to SMU after this command would be stalled as the
PMFW would not be in a state to take further job requests.

Technically this is right in case of S3 scenario. But, this might
not be the case during s0ix as the PMC driver would be the last
to send the SMU on the OS_HINT. If SMU gets a PrepareMp1ForUnload
message before the OS_HINT, this would stall the entire S0ix process.

Results show that, this message to SMU is not required during S0ix
and hence skip it.

Reviewed-by: Prike Liang <Prike.Liang@amd.com>
Signed-off-by: Shyam Sundar S K <Shyam-sundar.S-k@amd.com>
Acked-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>