drm/amd/pp: Clear smu response register before send smu message

smu firmware do not update response register immediately under
some delay tasks, we may read out the original value.

so need to clear the register before send smu message.

Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>