libhwcomposer: draw full layer if dirty region list is long.

Since blitting region by region by copybit is serial,
If a layer has long list of dirty regions, drawing full layer
is optimal. Make blit region same as display frame of layer.

CRs-fixed: 430607
Change-Id: I8f8acd2a785bdac9026c2d7ca77ee5caaf909f2b