drm/amdgpu/userq: optimize enforce isolation and s/r

If user queues are disabled for all IPs in the case
of suspend and resume and for gfx/compute in the case
of enforce isolation, we can return early.

Reviewed-by: Prike Liang <Prike.Liang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>