drm/amdgpu: Fix missing error check in suspend

amdgpu_device_suspend might return an error code since it can be called
from both runtime and system suspend flows. Add the missing return code
in case of a failure.

Reviewed-by: Oak Zeng <oak.zeng@amd.com>
Reviewed-by: Felix Kuehling <felix.kuehling@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Rajneesh Bhardwaj <rajneesh.bhardwaj@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>