freedreno: introduce fd_batch

freedreno: introduce fd_batch

Introduce the batch object, to track a batch/submit's worth of
ringbuffers and other bookkeeping.  In this first step, just move
the ringbuffers into batch, since that is mostly uninteresting
churn.

For now there is just a single batch at a time.  Note that one
outcome of this change is that rb's are allocated/freed on each
use.  But the expectation is that the bo pool in libdrm_freedreno
will save us the GEM bo alloc/free which was the initial reason
to implement a rb pool in gallium.

The purpose of the batch is to eventually facilitate out-of-order
rendering, with batches associated to framebuffer state, and
tracking the dependencies on other batches.

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