libweston: Notify client for change in content-protection status

The change in an output's content-protection may trigger a change in
the surface's content-protection status, and inturn the
content-protection available for the client.

This patch recomputes the content-protection level for a surface,
in case there is a change in content-protection level of an output,
showing the surface. In case of a change in the surface's
content-protection, the client associated with that surface is
notified.

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