iris: Do binder address allocations per-context, not globally.

iris: Do binder address allocations per-context, not globally.

iris_bufmgr allocates addresses across the entire screen, since buffers
may be shared between multiple contexts.  There used to be a single
special address, IRIS_BINDER_ADDRESS, that was per-context - and all
contexts used the same address.  When I moved to the multi-binder
system, I made a separate memory zone for them.  I wanted there to be
2-3 binders per context, so we could cycle them to avoid the stalls
inherent in pinning two buffers to the same address in back-to-back
batches.  But I figured I'd allow 100 binders just to be wildly
excessive/cautious.

What I didn't realize was that we need 2-3 binders per *context*,
and what I did was allocate 100 binders per *screen*.  Web browsers,
for example, might have 1-2 contexts per tab, leading to hundreds of
contexts, and thus binders.

To fix this, we stop allocating VMA for binders in bufmgr, and let
the binder handle it itself.  Binders are per-context, and they can
assign context-local addresses for the buffers by simply doing a
ringbuffer style approach.  We only hold on to one binder BO at a
time, so we won't ever have a conflicting address.

This fixes dEQP-EGL.functional.multicontext.non_shared_clear.

Huge thanks to Tapani Pälli for debugging this whole mess and
figuring out what was going wrong.

Reviewed-by: Tapani Pälli <tapani.palli@intel.com>