lmb: Treat a region which is a subset as equal

In various cases logical memory blocks are coalesced; As a result doing
a strict check whether memory blocks are the same doesn't necessarily
work as a previous addition of a given block might have been merged into
a bigger block.

Fix this by considering a block is already registered if it's a pure
subset of one of the existing blocks.

Signed-off-by: Sjoerd Simons <sjoerd@collabora.com>