msm: kgsl: Attach mem_entry once we have valid GPU address

kgsl_mem_entry_attach_process() adds new entry to the mem_idr list
without really having a valid GPU address.

This new entry can be used by other thread with GPUADDR_IN_MEMDESC()
and destroy it.

Get GPU address first and then add it to the mem_idr list.

Change-Id: I4d66cec754ca5315df3c9fe09644f55596c3c33c
Signed-off-by: Rajesh Kemisetti <rajeshk@codeaurora.org>