video: fix some comments in drivers/video/console/vgacon.c

Now vgacon_scrollback_startup() uses slab, not bootmem,
the comment above it is obsolete, so does __init_refok.

Signed-off-by: WANG Cong <amwang@redhat.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>