Fix draw race condition regression.

Change-Id: Idcf7c19c2467edf941a8a426078b6f7d7779ab12
Reviewed-on: https://swiftshader-review.googlesource.com/4943
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>