i386/PAE busdma: allow more bounce pages.

If i386 has more than 4G of memory, allow the same number of busdma
bounce pages as for amd64. In fact, in this case bouncing sometimes
is much heavier than on amd64.

Reviewed by: markj
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D18854