fbdev: dpu20: blocking mode set condition is modified

fbdev: dpu20: blocking mode set condition is modified

In the past, it was possible to set a blocking area in a bottom window
if the top window plane alpha value was 0. In this case, the top window
becomes transparent, and the blocking area of the window looks black.
 So it is modified that when the top window plane alpha value was zero,
blocking area isn't set in the bottom window.

Change-Id: I2340369331082c939f4d7013c002452affd836bc
Signed-off-by: hwangjae lee <hj-yo.lee@samsung.com>
Signed-off-by: ChiHun Won <chihun.won@samsung.com>