msm_fb: HDMI: HDCP: Start HDCP authentication as a work item

HDCP authentication should be kicked off as a scheduled work
item so as to ensure that MDP is already configured and frames
are being transmitted over the HDMI channel prior to the start
of the HDCP authentication procedure.

Change-Id: I27cab44babfc24d91a318b0641674f85910cc33f
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>