drm/amd/display: add workaround for riommu invalidation request hang

[Why]
When an riommu invalidation request come at the same time as a pipe is
disabled there can be a case where DCN cannot ACK the request if only
one VMID is setup in the inuse list.

[How]
Setup a second unused VMID will work around the issue.

Reviewed-by: Jun Lei <jun.lei@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Eric Yang <Eric.Yang2@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>