Clean ComposerClient cache on hotplug

On subsequent hotplug connected event for a display
SurfaceFlinger destroys the previous framebuffers and
recreates them. When the new buffers are created
ComposerClient still holds a handle to the old buffers and
they are not destroyed. This way the new framebuffers
may get allocated on non continuous memory causing garbled
screens for the user.

Bug: 160112047
Bug: 169255692
Test: 1. limit cma ion memory to 32 MB
      2. flash device
      3. plug hdmi out and in
      4. verify that the display image is not garbled
Change-Id: Idf7cdf7a070ffc83ecec34ac24c8a7d696f68aa6