radeon/winsys: add dma ring support to winsys v3

radeon/winsys: add dma ring support to winsys v3

Add ring support, you can create a cs for each ring. DMA ring is
bit special regarding relocation as you must emit as much relocation
as there is use of the buffer.

v2: - Improved comment on relocation changes
    - Use a single thread to queue cs submittion this simplify driver
      code while not impacting performances. Rational for this is that
      you have to wait for all previous submission to have completed
      so there was never a case while we could have 2 different thread
      submitting a command stream at the same time. This code just
      consolidate submission into one single thread per winsys.
v3: - Do not use semaphore for empty queue signaling, instead use
      cond var. This is because it's tricky to maintain an even number
      of call to semaphore wait and semaphore signal (the number of
      cs in the stack would for instance make that number vary).

Signed-off-by: Jerome Glisse <jglisse@redhat.com>