drm/vmwgfx: Fix error handling in get_node

ttm_mem_type_manager_func.get_node was changed to return -ENOSPC
instead of setting the node pointer to NULL. Unfortunately
vmwgfx still had two places where it was explicitly converting
-ENOSPC to 0 causing regressions. This fixes those spots by
allowing -ENOSPC to be returned. That seems to fix recent
regressions with vmwgfx.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Martin Krastev <krastevm@vmware.com>
Sigend-off-by: Roland Scheidegger <sroland@vmware.com>