msm: kgsl: Support user specified timeouts in adreno_waittimestamp

We have always allowed the user to specify a timeout in waittimestamp
but we never obeyed it. Reorganize the function to be less convoluted
and support a user specified timeout (or spin forever if 0 is passed).

Change-Id: Ic0dedbadbc368336d05b14d66829205631046515
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>