msm: mdss: Prevent modification to image roi when scaling

If pixel extension is enabled by scaler, and layer content is
deinterlaced, the modifications to the layers region of interest
to be used by mdp hw has already been modified. We should not
further changing these values.

Change-Id: I2c528b54ce07e90b66b56ef008b73b78e4227c9d
Signed-off-by: Terence Hampson <thampson@codeaurora.org>