gpu: ion: Refactor locking

Removes contention for lock between allocate and free by reducing
the length of time the lock is held for.  Split out a seperate
lock to protect the list of heaps and replace it with a rwsem since
the list will most likely only be updated during initialization.

Change-Id: I3017ccaa2126c7a24ab22b18e1ee553983977c37
Signed-off-by: Rebecca Schultz Zavin <rebecca@android.com>