x86: use brk allocation for DMI

Impact: use new interface instead of previous ad hoc implementation

Use extend_brk() to allocate memory for DMI rather than having an
ad-hoc allocator.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>