drm/msm: add HDCP helper functions to DRM HDMI driver

Add useful helper functions to the DRM HDMI driver
to be used later when HDCP functionality gets added.

Also add and initialize necessary members to the
SDE HDMI controller to facilitate easier integration
of DRM HDCP module.

Change-Id: I699f6685327e674a871a404fe5cf3adebe823d46
Signed-off-by: Abhinav Kumar <abhinavk@codeaurora.org>