msm: kgsl: Move timestamps inside the context structure

Store the timestamps inside the context rather than in a
list stored in the ringbuffer.  This allows for easier
maintanability as well as keeping all context data
centralized.

Change-Id: I0467d07be6c8bb9f062a81a40629c0288be7e868
Signed-off-by: Carter Cooper <ccooper@codeaurora.org>