drm/amdgpu: rework reserved VMID handling

Instead of reserving a VMID for a single process allow that many
processes use the reserved ID. This allows for proper isolation
between the processes.

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