drm/amdgpu: Disable GFX PG on CZ

It's causing problems with user mode queues and the HIQ, and can
lead to hard hangs during boot after programming RLC_CP_SCHEDULERS.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>