drm/powerplay: fix compilation warning

If (smu)->ppt_funcs->intf is not initialized then
smu_send_smc_msg(smu, SMU_MSG_ReadSerialNumBottom32, &bottom32) and
smu_send_smc_msg(smu, SMU_MSG_ReadSerialNumTop32, &top32) will not
touch bottom32 and top32.

Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>