anv: Skip reallocating the measurement BO when there are no events
This seems to happen a lot on secondary command buffers, where we're recycling a previous buffer that already had a measurement BO. If there are no events recorded, we don't need to worry about trashing anything there and can just reuse the BO we already have.
Nearly doubles the performance of Sascha Willem's multithreading demo with INTEL_MEASURE=batch,file=/dev/null set. Fixes similar issues in Dota 2 VK.