No need to burn an entire PAGE_SIZE BO for an event. And in particular
the pattern of allocate + immediate mmap is expensive in a VM.
Suballocating cuts down the # of times we do this in
dEQP-VK.api.command_buffers.execute_large_primary from 10000 to 157,
avoiding problems with the test running up against watchdog timeout.
Signed-off-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/33721>
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