msm: kgsl: Store process mem entries in a rbtree

The list of memory objects attached to a process gets searched quite
a lot during normal operation of the driver.  For processes with a
lot of memory allocations, the linear search through the list is O(N)
and uses a lot more CPU during critical loops than it should. Change
the mem entry list to a rbtree for faster search speeds.

Change-Id: Ic0dedbad1b25d9d77f56f93696b2fe933fbad333
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>