msm: kgsl: Change the active context tracker

The active context tracker that we bolted on the side of the
dispatcher was designed mainly to see if there was 1 OR more than 1
active context, not much more than that.  Since it is apparent that
we'll need to track up to 4 contexts and possibly more later the
algorithm needs to change.

The new algorithm puts all active contexts on a linked list  - every
time a context is used it is popped from the list and put on the
tip with an updated time. To count the number of active contexts
walk the list until you get a context with an active time older than
you are looking for.  You also can do other magic on the context,
like see if it matches up with a given command queue.

Change-Id: Ic0dedbad6be9fd1925121ee54e0000c42b089f44
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Prakash Kamliya <pkamliya@codeaurora.org>