drm/msm/hdcp: Avoid double authentication if first is still in progress

In scenarios where two streams become active in a short time period, HDCP
authentication may incorrectly be kicked off twice. This occurs due to the
HDCP state machine not indicating when authentication is already in
progress.

Update the dp_dispay HDCP state machine to only transfer to the
authenticating state once authentication has begun.

Change-Id: I38211203afe5127b80353c02072032a56e518900
Signed-off-by: Christopher Braga <cbraga@codeaurora.org>