Use list_first_entry_or_null() for first_region() and first_queue_range().

list_first_entry() expects the list is not empty, so first_region() and
first_queue_range() never return NULL.
Thus use list_first_entry_or_null() instead.
Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Santosh Shilimkar <ssantosh@kernel.org>