staging: ion: Move shrinker out of heaps

Every heap that uses deferred frees is going to need a shrinker
to shrink the freelist under memory pressure.  Rather than
requiring each heap to implement a shrinker, automatically
register a shrinker if the deferred free flag is set.
The system heap also needs to shrink its page pools, so add
a shrink function to the heap ops that will be called after
shrinking the freelists.

Cc: Colin Cross <ccross@android.com>
Cc: Android Kernel Team <kernel-team@android.com>
Signed-off-by: Colin Cross <ccross@android.com>
[jstultz: Resolved big conflicts with the shrinker api change.
Also minor commit subject tweak.]
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>