[S390] vt220 console: convert from bootmem to slab

The slab allocator is earlier available so convert the
bootmem allocations to slab/gfp allocations.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>