msm: kgsl: Don't use stale data when looking for active buffers

When searching for the active ringbuffer and indirect buffers to
snapshot, start with a GPU address of 0 if the data cannot be read
from the GPU registers.

Change-Id: Ib20064cb9f360ff295006da92308648d1d015126
Signed-off-by: Lynus Vaz <lvaz@codeaurora.org>