msm: kgsl: Remove superfluous NULL checks when reading timestamps

There is no way for the UMD to pass in NULL pointers for reading
timestamps. The KGSL driver also always has valid pointers
for storing the readpointer. Remove the checks that are not needed.

Change-Id: Id5f244a57a2b991a10b603ef7236193d4282fd0f
Signed-off-by: Carter Cooper <ccooper@codeaurora.org>