FFMPEG support was not getting enabled due to buggy logic. Cleaned that up. Now,
when PARAVIEW_ENABLE_FFMPEG is ON, we require VTK's FFMPEG support modules and
enable *.avi support. This replaces VTK_HAS_FFMPEG_SUPPORT with
PARAVIEW_ENABLE_FFMPEG in vtkPVConfig.h.

Change-Id: I047f1143857468542a448d90a0dc58b1be8a0d1a