drm/amdgpu: use new iterator in amdgpu_ttm_bo_eviction_valuable

Simplifying the code a bit.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20211005113742.1101-13-christian.koenig@amd.com