gpu: ion: Switch heap rbtree to a prio list

Switches the rbtree tree of heaps for a plist.  This significantly
simplifies the code and the list is small and is modified only at
first boot so the rbtree is unnecessary.  This also switches
the traversal of the heap list to traverse from highest to lowest
id's.  This allows allocations to pass a heap mask that falls
back on the system heap -- typically id 0, which is the common case.

Change-Id: I82617f7e410e1b4bae7111e933fd4f3e7fd13980
Signed-off-by: Rebecca Schultz Zavin <rebecca@android.com>
Git-commit: a910c8c45223c2a92e9da052231c2d118cc4d055
Git-repo: https://android.googlesource.com/kernel/common


[lauraa@codeaurora.org: Context differences in ion.c due to
additional debugging information added on the allocation path.
Need to adjust MSM specific code to use the new walking functions]
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>