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: Id10464dfe0d60cdcd64f29edfc94317d8e5ee251
Signed-off-by: Rebecca Schultz Zavin <rebecca@android.com>
Git-commit: 675a52aa0d89e8b6c0c05849627381d8a64b2b2b
Git-repo: https://android.googlesource.com/kernel/common


[lauraa@codeaurora.org: Context differences due to debugfs
differences. Need to adjust locking on MSM specific extensions]
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>