msm: kgsl: Clean-up gpuaddr allocated from kgsl gen_pool

All GPU Mapping which are done using kgsl_mmu_map_global
API, allocates gpu address from kgsl gen_pool. Only calling
kgsl_mmu_unmap for such mappings doesn't free up the gpuaddr
assigned from KGSL gen_pool. This results in Kernel bug when
KGSL gen_pool is destroyed. This changes makes sure that
all KGSL gen_pool GPU address are freed before destroying
kgsl gen_pool.

Change-Id: Iba46f18e7fe4fba661f5fe142f3470f61a31baa0
Signed-off-by: Anshuman Dani <adani@codeaurora.org>