enforce process isolation between graphics and compute via using the same reserved vmid.

v2: remove params "struct amdgpu_vm *vm" from
    amdgpu_vmid_alloc_reserved and amdgpu_vmid_free_reserved.
Signed-off-by: Chong Li <chongli2@amd.com>
Reviewed-by: Christian Koenig <Christian.Koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>