vt: panic while changing vt font
Set refcount for loader provided font to 1 to prevent this font
from being released (so we can reset to default).
As we get started with no memory allocator, we set up static font data
for font passed by loader (if there is any). At this time, we also must
set refcount 1, and refcount will get incremented in cnprobe() callback.
At some point the memory allocator will be available, and we will set up
properly allocated font data, but we should not disturb the refcount.