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