gpu: ion: use a list instead of a tree for heap debug memory map

Currently we use an rb tree to store information about the memory map
which gets passed to the heap print_debug functions. The reason for
using a tree instead of a simple list is to maintain sortedness as we
build the memory map. However, it can be necessary to store multiple
entries for the same address in the memory map since there can be
multiple clients with handles to the same buffer. This information is
interesting and useful but we currently can't store and display it since
the rb tree requires that the key used for sorting (the physical address
in this case) be unique. Fix this by replacing the rb tree with a linked
list. In order to maintain sorted output of the print_debug functions,
sort the list by physical address after fully building the list.

This also has the positive side-effect of simplifying the code and
making future print_debug methods less error-prone.

CRs-Fixed: 571918
Change-Id: I5b129fd809fb53c66042eab10d096238a34c2b20
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>