msm: kgsl: Allow the wait_timeout to be disabled

With the improved hang detection we don't technically need a upper
timeout bound for a process to wait in waitfortimestamp().  Allow
for a idle_timestamp of '0' to allow the loop to wait for ever.
The infrastructure for changing the idle_timeout still exists
so it can be changed to whatever value is appropriate for debugging
or testing.

CRs-fixed: 382366
CRs-fixed: 383999
Change-Id: Ic0dedbadf2c100eb56ed2338914575061776e725
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>