drm/amd/display: Blank stream before destroying HDCP session

[Why]
Stream disable sequence incorretly destroys HDCP session while stream is
not blanked and while audio is not muted. This sequence causes a flash
of corruption during mode change and an audio click.

[How]
Change sequence to blank stream before destroying HDCP session. Audio will
also be muted by blanking the stream.

Cc: stable@vger.kernel.org
Signed-off-by: Jaehyun Chung <jaehyun.chung@amd.com>
Reviewed-by: Alvin Lee <Alvin.Lee2@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>