This adds support for writing and reading back in a single
oneshot packet.  This is needed to send a tlb invalidation
and wait for ack in a single operation.

v2: squash the gfx ring stall fix
Reviewed-by: NHuang Rui <ray.huang@amd.com>
Reviewed-by: NChristian König <christian.koenig@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>
Signed-off-by: NEmily Deng <Emily.Deng@amd.com>