vm_phys_enqueue_contig: handle npages==0

By letting vm_phys_enqueue_contig handle the case when npages == 0,
the callers can stop checking it, and the compiler can stop
zero-checking with every call to ffs(). Letting vm_phys_enqueue_contig
call vm_phys_enqueue_contig for part of its work also saves a few
bytes.

The amd64 object code shrinks by 80 bytes.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D41154