msm: mdss: fix bug for validating pixel extension calculations

While validating pixel extension calculations from user space,
validate the over fetch pixels with image dimentions in both
horizontal and vertical directions.

Change-Id: I8e39e89d9ace8e26237b3b23ca375456abd804a1
Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>