msm: wfd: Modify enc_avg_latency to only measure the encode latency

Previously enc_avg_latency measured the time Video code took took to
encode a frame and the latency of the userspace client in picking up the
buffer.  Change the meaning just to measure encoder busy time.

CRs-Fixed: 647538
Change-Id: Ib7c30e51a267a0a2926096f13a933cdc94656453
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>