drivers/gpu/vga/vgaarb.c: add missing kfree

kbuf is a buffer that is local to this function, so all of the error paths
leaving the function should release it.

Signed-off-by: Julia Lawall <julia@diku.dk>
Cc: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>