msm: mdss: Unlock correct mutex in AD setup

The incorrect mutex for the BL file descriptor was being unlocked
in AD setup. Removed the incorrect unlock and replaced it with an
unlock on the correct mutex.

Change-Id: I276823c34d1183b69d7af06bae8b02cb7e1b56b1
Signed-off-by: Benet Clark <benetc@codeaurora.org>