drm/amd/pm: fix uninitialized variable warning

Check the return of function smum_send_msg_to_smc
as it may fail to initialize the variable.

Signed-off-by: Jesse Zhang <Jesse.Zhang@amd.com>
Reviewed-by: Tim Huang <Tim.Huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>