msm: kgsl: Do not free secure buffers without unlocking

Accessing secure buffer without unlocking can result
into EL1 exception. Make sure that secure buffers are
unlocked before we free it.

Change-Id: I83e070d3ae8db6a96ae0034f0b23ce14ed9aef7d
Signed-off-by: Hareesh Gundu <hareeshg@codeaurora.org>