vapostproc: Refactor gst_va_vpp_complete_caps_features()

vapostproc: Refactor gst_va_vpp_complete_caps_features()

gst_va_vpp_complete_caps_features() now receives the @feature_name to
add and return if @caps doesn't provide it.

So, instead of two nested loops, now the function is a single loop,
traversing @caps to find if each structure already contains the requested
@features_name.

It's important to add missing caps features with @caps, in order to
not lost information.

The function caller does the external loop by calling per each
available caps feature.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1024>