virgl: Work around possible memory exhaustion

Since we don't normally flush before performing copy transfers, it's
possible in some scenarios to use too much memory for staging resources
and start failing. This can happen either because we exhaust the total
available memory (including system memory virtio-gpu swaps out to), or,
more commonly, because the total size of resources in a command buffer
doesn't fit in virtio-gpu video memory.

To reduce the chances of this happening, force a flush before a copy
transfer if the total size of queued staging resources exceeds a certain
limit. Since after a flush any queued staging resources will be
eventually released, this ensures both that each command buffer doesn't
require too much video memory, and that we don't end up consuming too
much memory for staging resources in total.

Fixes kernel errors reported when running texture_upload tests in glbench.

Signed-off-by: Alexandros Frantzis <[email protected]>
Reviewed-by: Chia-I Wu <[email protected]>

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