msm: kgsl: Properly delete the context list node

Use the standard list APIs to delete the context node from the
master context list.  While the current code probably works, it is
risky to mess around with list internals.

Change-Id: Ic0dedbad00a1aa502a4d10feea77f9497abb473f
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>