iris: Move suballocated resources to a dedicated allocation on export

We don't want to export suballocated resources to external consumers,
for a variety of reasons.  First of all, it would be exporting random
other pieces of memory which we may not want those external consumers
to have access to.  Secondly, external clients wouldn't be aware of
what buffers are packed together and busy-tracking implications there.
Nor should they be.  And those are just the obvious reasons.

When we allocate a resource with the PIPE_BIND_SHARED flag, indicating
that it's going to be used externally, we avoid suballocation.

However, there are times when the client may suddenly decide to export
a texture or buffer, without any prior warning.  Since we had no idea
this buffer would be exported, we suballocated it.  Unfortunately, this
means we need to transition it to a dedicated allocation on the fly, by
allocating a new buffer and copying the contents over.

Making things worse, this often happens in DRI hooks that don't have an
associated context (which we need to say, run BLORP commands).  We have
to create an temporary context for this purpose, perform our blit, then
destroy it.  The radeonsi driver uses a permanent auxiliary context
stored in the screen for this purpose, but we can't do that because it
causes circular reference counting.  radeonsi doesn't do the reference
counting that we do, but also doesn't use u_transfer_helper, so they
get lucky in avoiding stale resource->screen pointers.  Other drivers
don't create an auxiliary context, so they avoid this problem for now.

For auxiliary data, rather than copying it over bit-for-bit, we simply
copy over the underlying data using iris_copy_region (GPU memcpy), and
take whatever the resulting aux state is from that operation.  Assuming
the copy operation compresses, the result will be compressed.

v2: Stop using a screen->aux_context and just invent one on the fly to
    avoid circular reference counting issues.

Acked-by: Paulo Zanoni <paulo.r.zanoni@intel.com> [v1]
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12623>

src/gallium/drivers/iris/iris_context.h

@@ -848,6 +848,7 @@ struct iris_context {
 
 struct pipe_context *
 iris_create_context(struct pipe_screen *screen, void *priv, unsigned flags);
+void iris_destroy_context(struct pipe_context *ctx);
 
 void iris_lost_context_state(struct iris_batch *batch);