asahi: Add batch tracking logic
We already have the notion of an agx_batch, which encapsulates a render
pass. Extend the logic to allow multiple in-flight batches per context, avoiding
a flush in set_framebuffer_state and improving performance for certain
applications designed for IMRs that ping-pong unnecessarily between FBOs. I
don't have such an application immediately in mind, but I wanted to get this
flag-day out of the way while the driver is still small and flexible.
The driver was written from day 1 with batch tracking in mind, so this is a
relatively small change to actually wire it up, but there are lots of little
details to get right.
The code itself is mostly a copy/paste of panfrost, which in turn draws
inspiration from freedreno and v3d.