bhyve: do not explicitly map fbuf framebuffer

Allocating a BAR will call baraddr which maps the framebuffer. No need
to allocate it explicitly on init.

Reviewed by: grehan
Sponsored by: Beckhoff Autmation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D32596

(cherry picked from commit 5085153ae4d55692415716e0ed03204bdf96d34a)