drm/amdgpu/pm: Check the return value of smum_send_msg_to_smc

[ Upstream commit 579f0c21 ]

Check the return value of smum_send_msg_to_smc, otherwise
we might use an uninitialized variable "now"

Signed-off-by: Ma Jun <Jun.Ma2@amd.com>
Reviewed-by: Tim Huang <Tim.Huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>