msm: kgsl: Optimize virtual memory allocation

Create a bitmap to track virtual memory allocations for
each pagetable and use this bitmap to efficiently allocate
virtual memory instead of walking the rb tree of the process.
Also, when checking for address collisions for a process
use rb tree macros to traverse the rb list when looking
for next address collisions.

CRs-Fixed: 672081, 675418
Change-Id: I9d0d9da773d112bb9f83b56714109237da66ce6e
Signed-off-by: Shubhraprakash Das <sadas@codeaurora.org>