xtensa: use generic vga.h

What xtensa has in asm/vga.h is the same as what can be found in
asm-generic/vga.h.  So use the latter header.

Link: http://lkml.kernel.org/r/20180907132219.12979-1-jslaby@suse.cz


Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Acked-by: Max Filippov <jcmvbkbc@gmail.com>
Cc: Chris Zankel <chris@zankel.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>