loader: do not autoselect smaller font than 8x16 with high res display

The VT screen buffer size is calculated based on our default
built in (8x16) font.

With high-resolution display, we want to use at least 8x16 font,
or we will have large unused areas on screen.

MFC after: 1 week

(cherry picke from commit a26f7358583174f2fe0df3e979f7b8b02069278c)