drm/amdgpu: fix concurrent VM flushes on Vega/Navi v2

Starting with Vega the hardware supports concurrent flushes
of VMID which can be used to implement per process VMID
allocation.

But concurrent flushes are mutual exclusive with back to
back VMID allocations, fix this to avoid a VMID used in
two ways at the same time.

v2: don't set ring to NULL

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: James Zhu <James.Zhu@amd.com>
Tested-by: James Zhu <James.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org