drm/radeon: Use two-ended allocation by size, v2

This decreases eviction by up to 20%, by improving the fragmentation
quality. No harm in normal cases that fit VRAM fully (PTS gaming suite).

In some cases, even the VRAM-fitting cases improved slightly (openarena, urban terror).

512kb was measured as the most optimal threshold for 3d workloads common to radeon.
Other drivers may need different thresholds according to their workloads.

v2: Nicer formatting
Signed-off-by: Lauri Kasanen <cand@gmx.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>