vt_vga: Use Write Mode 0 to draw group of 8 pixels using 3 or more colors

This replaces the method based on Write Mode 3, which required reads
from the video memory to load the latches.

MFC after:	1 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@270620 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f