drm/amdgpu: handle enforce isolation on non-0 gfxhub

Some chips have more than one gfxhub so check if we
are a gfxhub rather than just gfxhub 0.

Acked-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>