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: I715be6f4cf020a84ca4f1947c30ee3d2559fb523
Signed-off-by: Rebecca Schultz Zavin <rebecca@android.com>