msm: kgsl: Initialize pagetable pointer to NULL on declaration

Initialize local pagetable pointer to NULL in the function
kgsl_mem_entry_attach_process(). This avoids GPU page faults in
case GPU address for the memory entry does not exist.

Change-Id: Ibb37f085bc38b025e5c7305e662c5d6d5ab8106c
Signed-off-by: Lokesh Batra <lbatra@codeaurora.org>