msm: kgsl: Add a ftrace event for GPU busy

Log a ftrace event when the GPU statistics are collected so that
we can keep track of GPU busy along with the other statistics
in the ftrace stream.

Change-Id: Ic0dedbadb58b9bef6e8669a7634bf9ca87073bcb
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>