msm: kgsl: Replace the sorted dispatcher pending list

Since the beginning of the dispatcher all pending draw contexts have been
attached to one big sorted priority list complete with locks. This is
overweight, requires a number of not so pretty workarounds for requeuing,
and (most importantly) plist is not exported for modules.

Replace the master plist with a set of 16 lists, one for each priority.
By using llist we can avoid the lock and easily solve the circular
requeuing problems we had before. The result should be cleaner, faster
and module friendly.

Change-Id: Ic0dedbad195a58d791a3583d244e39a14f2de941
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>