equalizer: fix dynamic changes on bands
equalizer: fix dynamic changes on bands

When we are in passthrough, the transform function doesn't run and if the
passthrough check is in this function it will never be deactivated. Fix this by
checking directly whenever a gain is changed.

Also set the passthrough to TRUE at init because the gains default to 0, so we
can passthrough until any gain property is changed.

https://bugzilla.gnome.org/show_bug.cgi?id=748068

sebras/gst-plugins-good,Kurento/gst-plugins-good,sebras/gst-plugins-good,stfl/gst-plugins-good,chamois94/gst-plugins-good,froggatt/gst-plugins-good-m,shelsonjava/gst-plugins-good,stfl/gst-plugins-good,Kurento/gst-plugins-good,froggatt/gst-plugins-good-m,ikonst/gst-plugins-good,kittee/gst-plugins-good,vatavuserban/gst-plugins-good,ikonst/gst-plugins-good,StreamUtils/gst-plugins-good,hizukiayaka/gst-plugins-good,froggatt/gst-plugins-good-m,froggatt/gst-plugins-good-m,hizukiayaka/gst-plugins-good,Kurento/gst-plugins-good,shelsonjava/gst-plugins-good,pexip/gst-plugins-good,rawoul/gst-plugins-good,ikonst/gst-plugins-good,GrokImageCompression/gst-plugins-good,vatavuserban/gst-plugins-good,hizukiayaka/gst-plugins-good,kittee/gst-plugins-good,chamois94/gst-plugins-good,GStreamer/gst-plugins-good,StreamUtils/gst-plugins-good,rawoul/gst-plugins-good,shelsonjava/gst-plugins-good,Kurento/gst-plugins-good,vatavuserban/gst-plugins-good,rawoul/gst-plugins-good,ikonst/gst-plugins-good,StreamUtils/gst-plugins-good,GStreamer/gst-plugins-good,kittee/gst-plugins-good,rawoul/gst-plugins-good,GrokImageCompression/gst-plugins-good,pexip/gst-plugins-good,vatavuserban/gst-plugins-good,GStreamer/gst-plugins-good,kittee/gst-plugins-good,chamois94/gst-plugins-good,StreamUtils/gst-plugins-good,Kurento/gst-plugins-good,chamois94/gst-plugins-good,GrokImageCompression/gst-plugins-good,sebras/gst-plugins-good,shelsonjava/gst-plugins-good,stfl/gst-plugins-good,pexip/gst-plugins-good,sebras/gst-plugins-good,stfl/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,GrokImageCompression/gst-plugins-good,hizukiayaka/gst-plugins-good