msm: kgsl: Set a failed GMU register read to 0

Set the return variable to 0 instead of 0xDEADBEEF.
If the register read failed and the code then checks the
returned value against certain bits being set, it could
have false positives and perform the wrong actions.

Change-Id: I3f3a09f8b32ce9cf7b95378a4232d29aaa1629f8
Signed-off-by: Carter Cooper <ccooper@codeaurora.org>