msm: kgsl: Remove use of uninitialized variable

During certain code execution paths dev_ab can be assigned
not initialized value.

CRs-Fixed: 1074208
Change-Id: I330a50f6d171497e5b6d3e44e8ce2b09f2d644e3
Signed-off-by: Oleg Perelet <operelet@codeaurora.org>