s390/vx: Convert cpu_has_vx() to cpu feature function

Instead of having a private cpu_has_vx() implementation use the new common
cpu feature method. Move the facility detection to the decompressor so it
matches all other cpu features.

Reviewed-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>