x86: enable DMI and DMI_SYSFS

Enable support for DMI decoding in the kernel so that we can access
manufacturer and model information via sysfs.

Also remove redundant per-subtarget DMI overrides and preset a few
previously unset symbols popping up due to the now enabled DMI support.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>