mediatek: Don't compose empty dirty regions on virtual displays

When performing virtual composition this will create more frames than
are actually required, since there is no dirty region.

On the mediatek platform, this problem is more noticeable because the
gralloc uses a private pixel format that CTS does not expect and it
analyzes each frame from the virtual composition, eventually timing out
and failing.

This fixes the following tests:

android.media.cts.EncodeVirtualDisplayWithCompositionTest#testRendering800x480Locally
android.media.cts.EncodeVirtualDisplayWithCompositionTest#testRendering800x480LocallyWith3Windows
android.media.cts.EncodeVirtualDisplayWithCompositionTest#testRenderingMaxResolutionRemotely

Change-Id: Icdc2c3337f2cdf4aa27d435cf768649793cfdeca