fbdev: msm: populate pdev entry for debug structure

Platform device structure is populated in debug init
in mdp3. This entry helps in allocating memory, for
xlog buffers output to be dumped in memory.

Change-Id: Iafbce324ce3b0bf098c0c6aa9b60935a99f94831
Signed-off-by: Jayaprakash <jmadiset@codeaurora.org>