vt_vga: add a timeout while waiting for vertical retrace

On one of my systems FreeBSD will fail to boot because vt_vga gets stuck
waiting for the vertical retrace if there's no monitor attached. Fix this by
adding a timeout and exiting if the vertical retrace times out.

Sponsored by: Citrix Systems R&D
Reviewed by: emaste, dumbbell
Differential Revision: https://reviews.freebsd.org/D2397