Change override displayFrame to mBounds

Shrinks the area of the overridebuffer sampled by the dpu to the
boundaries of the layer(s) flattened in that buffer.

Bug: b/192058025

Test: composition unit tests, confirm dpu bandwidth is reduced,
confirm secondary displays are still working.

Change-Id: I46d0401d3a08a9c9019f4e1fef1b301e91e6d241