drm/amdgpu: add amdgpu_ttm_bo_eviction_valuable callback

This way we can correctly check split VRAM buffers as well.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>