msm: kgsl: Detach page table first for every close

Before deleting memory entries, detach page table from the list
for last fd close of a process. This will keep one to one mapping
between process private and page table.This fixes the issue where
new allocations under a new process structure do not conflict
with addresses mapped in older pagetable.

CRs-Fixed: 785064
Change-Id: I9e21fee93d7dee889199a3dc61f16cd8797b10bf
Signed-off-by: Rajeev Kulkarni <krajeev@codeaurora.org>