gpu: ion: Attempt heaps in the order given

A previous change (47b9888cc80b224b3d341227224c44b3c884d28a
"gpu: ion: Switch heap rbtree to a prio list") changed the
priority of the heaps to attempt higher numbers first. The
current setup of heaps assumes that lower numbers will be tried
first. Rather than break the userspace ABI once again to match
what was done previously, change the heap priority back.

Change-Id: Ibb7664e11c0b9037f6968e0766486936b3d42e16
CRs-Fixed: 502722
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>