msm: kgsl: Hold the plist spinlock to protect nodes

Even though we copy the plist to process the pending contexts the
individual nodes might need to be spinlock protected against
outsiders.

Change-Id: Ic0dedbad509e9b37b5bee4b4828561950ad2f73e
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>