gst/equalizer/gstiirequalizer.*: Update the filter coefficients only when needed in the transform_ip function and cor...

Original commit message from CVS:
* gst/equalizer/gstiirequalizer.c:
(gst_iir_equalizer_band_set_property), (gst_iir_equalizer_init),
(setup_filter), (set_passthrough), (update_coefficients),
(gst_iir_equalizer_compute_frequencies),
(gst_iir_equalizer_transform_ip):
* gst/equalizer/gstiirequalizer.h:
Update the filter coefficients only when needed in the transform_ip
function and correctly set the element into passthrough mode if the
gain of all bands is 0.