drm/amdkfd: Avoid null pointer in SMI event

Power Management IP is initialized/enabled before KFD init. When a
thermal throttling happens before kfd_smi_init is done, calling the KFD
SMI update function causes a stack dump by referring a NULL pointer (
smi_clients list). Check if kfd_init is completed before calling the
function.

Signed-off-by: Amber Lin <Amber.Lin@amd.com>
Reviewed-by: Mukul Joshi <mukul.joshi@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>