libweston: Compute current protection for weston_output and weston_head

The actual protection status for a given weston_head depends upon the
corresponding drm_head's connector HDCP properties. On the other hand,
the actual protection for a weston_output is the minimum of the
protection status of its attached heads.
As a head's protection changes, the current protection of the output
to which the head is attached is recomputed.

This patch adds the support to keep track of the current
content-protection for heads and the outputs.

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>