msm: mdss: fix secure buffer ref counting

Fix ref counting of the secure buffers. This change releases
the ref count on the secure buffers during the free operation.
This allows the buffers to be ref counted until hw has done
fetching from the buffers.

CRs-Fixed: 2012323
Change-Id: Ic24c39e75d502ed73453d0c6d78fd76448737697
Signed-off-by: Abhijit Kulkarni <kabhijit@codeaurora.org>