drm/amdgpu: minor PRT turnoff fix (v2)

When two VMs stop using PRT support at the same time we might
not disable it in the right order otherwise.

v2: agd: rebase

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