drm/amdgpu: move VM eviction decision into amdgpu_vm.c

When a page tables needs to be evicted the VM code should
decide if that is possible or not.

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