On Vega20 and other pre-production GPUs, powerplay is not enabled yet.
Check for NULL pointers before calling pp_funcs function pointers.

Also affects Kaveri.

CC: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Tested-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org