mdacon: rework dependency list

[ Upstream commit 5bbcc7645f4b244ffb5ac6563fbe9d3d42194447 ]

mdacon has roughly the same dependencies as vgacon but expresses them
as a negative list instead of a positive list, with the only practical
difference being PowerPC/CHRP, which uses vga16fb instead of vgacon.

The CONFIG_MDA_CONSOLE description advises to only turn it on when vgacon
is also used because MDA/Hercules-only systems should be using vgacon
instead, so just change the list to enforce that directly for simplicity.

The probing was broken from 2002 to 2008, this improves on the fix
that was added then: If vgacon is a loadable module, then mdacon
cannot be built-in now, and the list of systems that support vgacon
is carried over.

Fixes: 0b9cf3aa ("mdacon messing up default vc's - set default to vc13-16 again")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Helge Deller <deller@gmx.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>