drm/msm/dsi-staging: Free memory to prevent memleak

De-allocate command packet buffers while destroying
command objects. This fixes the memory leak issue
observed while sending DCS commands for panel ROI update.

Change-Id: I39ca6d288951d61057febcb5631165a20747143f
Signed-off-by: Amine Najahi <anajahi@codeaurora.org>