msm: kgsl: Defer adding the mem entry to a process

If we add the mem entry pointer in the process mem_idr too early
other threads can do operations on the entry by guessing the ID
or GPU address before the object gets returned by the creating
operation.

Allocate an ID for the object but don't assign the pointer until
right before the creating function returns ensuring that another
operation can't access it until it is ready.

CRs-Fixed: 1002974
Change-Id: Ic0dedbadc0dd2125bd2a7bcc152972c0555e07f8
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>