drm/amdgpu/gfx11: only enable CP GFX shadowing on SR-IOV

This is only required for SR-IOV world switches, but it
adds additional latency leading to reduced performance in
some benchmarks.  Disable for now on bare metal.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>