drm/vmwgfx: Return 0 when gmrid::get_node runs out of ID's

If it's not a system error and get_node implementation accommodate the
buffer object then it should return 0 with memm::mm_node set to NULL.

v2: Test for id != -ENOMEM instead of id == -ENOSPC.

Cc: <stable@vger.kernel.org>
Fixes: 4eb085e4

 ("drm/vmwgfx: Convert to new IDA API")
Signed-off-by: Deepak Rawat <drawat@vmware.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>