gallium/radeon: lower memory usage during texture transfers

gallium/radeon: lower memory usage during texture transfers

This improves throughput by keeping TTM overhead down.

Some piglit tests such as texelFetch and streaming-texture-leak will
use less memory now.

v2: use gart_size / 4 as the threshold

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>