msm: kgsl: Prevent race conditions when freeing memory

Multiple threads can call an ioctl to free a memory region.
Only one of these threads must be allowed to free memory and
the rest can return without freeing. Add a new pending flag
which is tested and set in a critical section guarded by
spinlock to prevent multiple ioctl threads from freeing the
same memory.

Also, a thread could be freeing a memory region that is
in use by another thread. Ensure that the detachment of the
memory from the process list and freeing of the memory always
happens in the same thread. This will prevent a situation where
the memory is being used by a thread which is detached from the
process list and is yet to be freed.

Seperate the selection of gpuaddr and mapping into pagetable. The
gpuaddr assignment needs to be done with process memory lock held
in a critical section but the mapping into pagetable can be done
separately without holding memory lock.

CRs-Fixed: 495144
Change-Id: Idf85fbd4bca29c18597f4b0e737c207f002ab266
Signed-off-by: Shubhraprakash Das <sadas@codeaurora.org>