drm/msm: configure AV mute before disabling encryption

In current implementation, AV mute packet is sent after
disabling the encryption. This can cause artifacts on the
sink when re-authentication is going on.

Send the AV mute packet to the sink before disabling
encryption so that no visual artifacts are observed
during re-authentication.

Change-Id: Ifbe656691b3750a76fbd48a776ba660ebbe5f65a
Signed-off-by: Abhinav Kumar <abhinavk@codeaurora.org>