s390/tty3270: add support for diag 8c

The current code uses diag210 to infer the 3270 geometry from the
model number when running on z/VM. This doesn't work well as almost
all 3270 software clients report as 3279-2 with a custom resolution.
tty3270 assumes it has a 80x24 terminal connected because of the -2
suffix. Use diag 8c to fetch the realy geometry from z/VM.

Note that this doesn't allow dynamic resizing, i.e. reconnecting to
a z/VM session with a different geometry.

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Tested-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>