The query of all the supported formats for a VA config were only used by the
postprocessor (vaapifilter). But, in order to enable the vaapidecoder to
negotiate a suitable raw format with downstream, we need to query these
formats against the decoder's config.

This patch is the first step: moves the code in filter's ensure_image() to a
generic gst_vaapi_get_surface_formats() in vaapiutils_core, so it can be
shared later by the decoder.

https://bugzilla.gnome.org/show_bug.cgi?id=752958