msm: kgsl: Do not return an error on NULL gpu address

If a NULL gpu address is passed to snapshot object tracking
function then do not treat this as an error and return 0. NULL
objects may be present in an IB so just skip over these objects
instead of exiting due to an error.

Signed-off-by: Shubhraprakash Das <sadas@codeaurora.org>
Change-Id: Ic253722c58b41f41d03f83c77017e58365da01a7