gpu: ion: Also shrink memory cached in the deferred free list

When the system is low on memory, we want to shrink any cached
system memory ion is holding.  Previously we were shrinking memory
in the page pools, but not in the deferred free list.  This patch
makes it possible to shrink both.  It also moves the shrinker
code into the heaps so they can correctly manage any caches they
might contain.

Signed-off-by: Rebecca Schultz Zavin <rebecca@android.com>
Change-Id: I177f587f999a5220eddbf7af94745aae736cac75