fbdev: save the activate field before calling fb_check_var()

Some drivers may reset the var->activate field on fb_check_var(). This can
lead to undefined behavior. For example, doing fbset -a <option> with vga16fb
will only modify the active console instead of modifying all.

Signed-off-by: Antonino Daplas <adaplas@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>