vga_switcheroo: don't toggle-switch devices

If the requested device is already active, ignore the request.

This restores the original behaviour of the interface. The change was
probably an unintended side effect of

commit 66b37c67

 vga_switcheroo: split switching into two stages

which did not take into account to duplicate the !active check in the split-off
stage2.

Fix this by factoring that check out of stage1 into the debugfs_write routine.

References: https://bugzilla.kernel.org/show_bug.cgi?id=34252
Reported-by: Igor Murzov <e-mail@date.by>
Tested-by: Igor Murzov <e-mail@date.by>
Signed-off-by: Florian Mickler <florian@mickler.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>