msm: mdss: Fix null pointer dereference and unintialisation of variables

Add NULL checks before accessing potential null pointers. Also, fix
uninitialised variables in MDP driver as they can be accessed with
incorrect values.

Change-Id: Id8a551fd3bc147c800ee1c98c2bc2b6f3bf6d380
Signed-off-by: Krishna Manikandan <mkrishn@codeaurora.org>