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.
(cherry picked and squashed 93ebd6307efeb95a29bc614edd0c67c2af850e98 and
1912d2b15e6d6f4a1d8c3886b03ab30d2f21f3fd)