msm: kgsl: Fix race condition during mem_entry detach

kgsl_mem_entry_detach_process() makes gpuaddr to NULL and then
removes the entry from mem_idr list.

Sometimes this can cause kgsl_sharedmem_find() to return the
same entry for a different gpuaddr/thread if it satisfies
GPUADDR_IN_MEMDESC().

To avoid this, first remove the entry from mem_idr list and
proceed with unmap/untrack calls.

Change-Id: Ib9f2bc0fdaecd394735dd61b18fdc7de57aa3748
Signed-off-by: Rajesh Kemisetti <rajeshk@codeaurora.org>