empty include/asm-generic/vga.h

all places that use anything defined in it (vgacon, mdacon and
vga16fb) are built only on architectures that have all that
stuff in their native asm/vga.h

allows to kill stub asm/vga.h on sh, while we are at it...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>