fbcon: use kvmalloc() for scrollback buffer

Scrollback frame buffer is rather big - 32K,
so it requires 3rd order page, so let's use kvmalloc() instead of
ordinary kmalloc() for it.

Signed-off-by: Konstantin Khorenko <khorenko@virtuozzo.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>