libs: encoder: h265: add refs property

libs: encoder: h265: add refs property

Users can provide the number of reference frame by this property,
which is exaclty same as h264.

The value of the property will be considered as the number of
reference picture list0 and will add 1 reference frame more to the
reference picture list1 internally if b-frame encoding.

If the value provided is bigger than the number of refrence frames
supported in the driver, it will be lowered.

The maximum value is aligned to the value of the driver supported now.

https://bugzilla.gnome.org/show_bug.cgi?id=783804