msdk: encoder: h265: generalize the behavior of "i-frames" property

msdk: encoder: h265: generalize the behavior of "i-frames" property

We have the property "i-frames" to set the IDR interval in a
gop. Unfortunately MSDK HEVC encoder behaves bit differently
for IdrInterval field, IdrInteval == 1 indicate every
I-frame should be an IDR (which is IdrInterval == 0 for other codecs),
IdrInteval == 2 means every other I-frame is an IDR
(which is IdrInterval == 1 for other codecs) etc.
So we generalize the behaviour of property "i-frames" by
incrementing the value by one in each case (only for HEVC).

https://bugzilla.gnome.org/show_bug.cgi?id=791637