sparc: get rid of asm/vga.h

The only thing we are using it for on sparc is telling vt_buffer.h
to pick what it would pick by default anyway - we are not accessing
any VRAM here...

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