msm: kgsl: Don't complain if we can't find an IB with a zero GPU address

When grabbing the snapshot don't complain if we can't find an IB at
address 0x0000000000000000.  Obviously such a thing doesn't exist so
don't even bother trying.

Change-Id: Ic0dedbadde29e16185f7a7d3e4d3b1d620697d3a
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>