msm: kgsl: don't clear gpuaddr when unmapping global mappings

Memory mapped through kgsl_mmu_map_global() is supposed to have
the same gpu address in all pagetables. And the memdesc will
persist beyond the lifetime of any single pagetable.
Therefore, memdesc->gpuaddr should not be zeroed for these
memdescs.

Change-Id: I0f46aaee2b9e87f839e78b7978cdf1bb4239d6f5
Signed-off-by: Jeremy Gebben <jgebben@codeaurora.org>