drm/msm: set avmute only if encryption is enabled

When HDCP authentication continuously fails there is no
need to set avmute packet for clear content because
there is no need to block the content in that case.

Check if encryption was enabled before setting avmute.

Change-Id: I7ece6538d36e0b0c9022e5993714f49275ed7db2
Signed-off-by: Abhinav Kumar <abhinavk@codeaurora.org>