drm/msm: add HDCP_VERSION property for connector

Expose a HDCP_VERSION property for the connector.
This property is used by the userspace to decide the
proper handling of secure streams based on HDCP version
of the sink.

This also guarantees that sinks do not receive
secure content higher than their capability.

Change-Id: Ia4ada2f26a5cde088b24c8792c10c9f9097602b1
Signed-off-by: Abhinav Kumar <abhinavk@codeaurora.org>