drm/amd/display: Refactor HDCP encryption status update

[Why]
The old way was to poll PSP and update the properties. But due to a
limitation in the PSP interface this doesn't work for MST.

[How]
According to PSP if set_encryption return success, the link is encrypted
and the only way it will not be is if we get a link loss(which we handle
already).

So this method should be good enough to report HDCP status.

Signed-off-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>