drm/amd/display: Add comments to hdcp property change code

[Why]
These comments are helpful in understanding which case each if
statement handles.

[How]
Add comments for state transitions (9 possible cases)

Signed-off-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Reviewed-by: Zhan Liu <zhan.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>