s390: use generic vga.h header file

The generic vga.h contains a couple of defines, which do no harm on
s390. Therefore use the generic version and git rid of the s390
specific empty header file.

Suggested-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>