vm_phys_init: Quiet unused but set warnings about npages.

npages is used in two optional cases:

• to conditionally create a separate DMA32 free list

• to index vm_page_array for VM_PHYSSEG_SPARSE

Add in more #ifdef's around npages statements.

Reviewed by: alc, markj
Differential Revision: https://reviews.freebsd.org/D34887