drm/msm/sde: cleanup handling of secure buffer in planes

This change handles the fb_translation mode property cleanly.
Plane reads this property only if the dirty bit is set. To accomplish
the correct HW programming, this change separates
the hw programming of the secure buffers in a separate function.

CRs-Fixed: 2064272
Change-Id: I05bcb6addabdedfed22c16854372eea5a61f1ade
Signed-off-by: Abhijit Kulkarni <kabhijit@codeaurora.org>