mesa: unreference zombie buffers when creating buffers to lower memory usage

This fixes an issue where one context only creates buffers while another context only destroys buffers. Only the creating context can release its buffers and the destroying context only turns them into zombie buffers.

This fix makes the creating context release its zombie buffers. It's not a plot from an apocalyptic movie.

Fixes: e014e3b6 "mesa: don't count buffer references for the context that created them" Closes: #4840 (closed)