Add workaround for vt efifb's early use of PHYS_TO_DMAP

In vt_efifb_init the framebuffer's physaddr is passed to PHYS_TO_DMAP
before the DMAP is setup. The result is not actually accessed until
after the mapping is setup, though. Loosen the assertion in PHYS_TO_DMAP
for now, to allow use when dmaplimit == 0.

Reviewed by:	kib
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D1142


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274382 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f