msm: mdss: reset cdm pointer when ctl is destroyed

Control structure is reused between interface and
rotator. It may possible that rotator will reuse the
control data structure previously used by writeback
interface which has cdm block allocated. It is safe
to reset the cdm pointer during control free to
avoid cdm setup during writeback format setup in
rotator scenario.

Change-Id: I7bef9893d6034ac3985c8808a1defeb15f0cf3fd
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>