sparse buffers are not cpu-readable, so any mapping requires that they
use a staging buffer, either the stream uploader for writes or a manual
copy for readback
future work here should attempt to resolve two perf issues:
* sparse allocations should be allocated dynamically using a suballocator
or some other, more useful strategy
* readback shouldn't allocate a huge staging buffer
Reviewed-by: Dave Airlie <airlied@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10509>
Mike Blumenkrantz
c90246b682