libs: encoder: h264: add multi reference support

libs: encoder: h264: add multi reference support

Using num_ref_frames provided and the result of the Query
VAConfigAttribEncMaxRefFrames, it determines the size of reference list
and perform encoding with multi reference frames as the following:

1\ The num_ref_frames is being considered as the number of
reference picture list0
2\ Encoder adds 1 reference frame more to the reference picture list1
internally if b-frame encoding.
3\ If num_ref_frames is bigger than the number of refrence frames
supported in the driver, it will be lowered.

https://bugzilla.gnome.org/show_bug.cgi?id=783803