dga: Make shutdown less magical and/or terrifying

DGAShutdown() walks every screen and attempts to reset the mode.  That's
maybe a reasonable thing to do, although the explicit loop is certainly
a bad smell.

In ddxGiveUp it's called after we've torn down the vga arbiter - and in
fact most of the rest of screen state - which is... very very bad.  The
other place it's called is from the Control-Alt-BackSpace handler, where
we don't even attempt to do vga arb setup, and where in any case we're
going to escape the main loop eventually anyway.

Move all that cleanup work inside DGACloseScreen. This means it happens
earlier in server teardown than previously, but not in a way you're ever
going to be upset about.

Signed-off-by: Adam Jackson <ajax@redhat.com>