msm: kgsl: Add a ftrace event for register writes

It is interesting to know when and how we program the hardware so
add a tracepoint to monitor register writes.  Register reads are
not traced since they are done much more frequently and are somewhat
less interesting from a debug perspective.

Change-Id: Ic0dedbad8ee85f154b9ba8731fe2c14603351243
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>