drm/amdgpu: fix TTM move entity init order

We are initializing the entity before the scheduler is actually
initialized.

This can lead to all kind of problem, but especially NULL pointer deref
because of Nayan's scheduler work.

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