freedreno/a6xx: Let the GPU track streamout offsets

freedreno/a6xx: Let the GPU track streamout offsets

The GPU writes out streamout offsets as it goes to the FLUSH_BASE
pointer.  We use that value with CP_MEM_TO_REG when appending to the
stream so that we don't have to track the offsets with the CPU in the
driver.  This ensures that streamout continues to work once we enable
geometry and tessellation shader stages that add geometry.

Reviewed-by: Rob Clark <robdclark@gmail.com>