msm: mdss: KW fixes in MDP driver

Fixes to validate input arguments and return
values of functions.

Change-Id: I3ddef7d944bcdcfff027a57f5e0e4ec121b08104
Signed-off-by: Nirmal Abraham <nabrah@codeaurora.org>
Signed-off-by: Raghavendra Ambadas <rambad@codeaurora.org>