drm/msm/sde: avoid detaching non-sec context bank during secure-ui

After the hardware enhancement for secure-display which allows
only secure data to be transferred during secure-display, the
non-secure context banks can co-exist during the session and
may be used for non-data related purpose. With this in effect,
there is no necessity for unmapping the non-sec buffers before
entering the secure session. Support backward compatibility, so
targets without this feature can still work the old way.

Change-Id: I54e0f67320e7680525f429934c30ca4179ac1000
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>