msm: mdss: fix display corruption on 720p panel

The MDP dma source size should match with the panel
resolution, instead of using the overlay size which
can be different from the panel size.

Change-Id: I4c9f8ea4e0d12e739646fe3d67dad299d3b3245b
Signed-off-by: Xiaoming Zhou <zhoux@codeaurora.org>