drm/amdgpu: rename enforce isolation variables

Since they will be used for both KFD and KGD user queues,
rename them from kfd to userq.  No intended functional
change.

Acked-by: Sunil Khatri <sunil.khatri@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>