x86: Do not reserve brk for DMI if it's not going to be used

This will save 64K bytes from memory when loading linux if DMI is
disabled, which is good for embedded systems.

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@holoscopio.com>
LKML-Reference: <1265758732-19320-1-git-send-email-cascardo@holoscopio.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>