drm/amdgpu: reuse VMIDs already assigned to a process

If we don't need to flush we can easily use another VMID
already assigned to the process.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>