Iris: piglit streaming-texture-leak test consumes large amounts of memory

This test iterates 5k times creating and deleting a large texture. It can run a system with 8GB Ram out of memory when running piglit tests in parallel, often resulting in gpu hang.

This is probably due to 0b6693a3, which increases the memory pressure of each iteration.

Iris may be able to check if large amounts of memory is ready to be freed when allocating new texture buffers. This test does not represent a realistic use case, and is not necessarily a reason to optimize the behavior.