[Intel-gfx] IVB page flipping fixes (hopefully final)

[Jesse Barnes]

This set addresses comments from Dan and Chris, namely:
  - pin & fence the new fb on the correct ring for IVB
  - cleanup gen2/3 wait code by putting it in gen2/3 wait functions
  - remove dead code
  - fix a race in the split code with the interrupt handler and flip pending

Thanks,
Jesse