msm_fb: display: Fix division by zero error in MDP downscale check

Fix division by zero error in MDP downscale validation by rejecting
destination rectangle width and height params with 0

Change-Id: Ia8c145e0bf00a97ca376f1ec98aa6926c9d7856c
Signed-off-by: Ravishangar Kalyanam <rkalya@codeaurora.org>