drm/msm/dsi-staging: handle dsi buffers during secure transitions

All dsi buffers should be unmapped before transitioning from
non-secure to secure, as all the context banks will be detached
and the buffers should be mapped back when transitioning from
secure to non-secure to resume normal DSI operations. Register
to msm_gem call back api to get callbacks during transitions
and handle the dsi buffers accordingly.

Change-Id: Ie6435248479e442406855a8cea0e4f2ab0c11845
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>