amdgpu: add amdgpu_stress utility v2

Simple yet useful command line utility to simulate memory pressure.

Already found quite a number of problems in TTM with that.

v2: replace spaces with tabs

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>