vt_vga: fix colour in pixel blocks with more than 4 colours

VGA hardware provides many different graphics and data access modes,
each with different capabilities and limitations.

VGA vt(4) graphics mode operates on blocks of pixels at a time. When a
given pixel block contains only two colours the vt_vga driver uses write
mode 3. When the block contains more than two colours it uses write
mode 0. This is done because two-colour write mode 3 is much more
efficient.

In practice write mode 3 is used most of the time, as there is often a
single foreground colour and single background colour across the entire
console. One common exception requiring the use of mode 0 is when the
mouse cursor is drawn over a background other than black, as we need
black and white for the cursor in addition to the background colour.

VGA's default 16-colour palette provides the same set of colours as the
system console, but in a different order. Previously we configured a
non-default VGA palette that had the same colours at the same indexes.
However, this caused anything drawn before the kernel started (drawn by
the loader, for instance) to change colours once the kernel configured
the new, non-default palette.

In 5e251aec8636 we switched to leaving the default VGA palette in place,
translating console colour indexes to VGA colour indexes as necessary.
This translation was missed for the write mode 0 case for pixel blocks
with more than two colours.

PR: 261751
Reviewed by: adrian
MFC after: 1 week
Fixes: 5e251aec8636 ("vt(4): Use default VGA palette")
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34412