drm/ttm: cleanup ttm_mem_type_manager_func.get_node interface v3

Instead of signaling failure by setting the node pointer to
NULL do so by returning -ENOSPC.

v2: add memset() to make sure that mem is always initialized.
v3: drop memset() only set mm_node = NULL, move mm_node init in amdgpu

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Link: https://patchwork.freedesktop.org/patch/373181/