vaapi: Handle when no encoders/decoders available (on nouveau)

Nouveau driver currently only exposes postproc entry. But vaapidecodebin is registered independent if there are decoders or not, exposing a segmentation fault.

This patch removes the encoder/decoder/codec arrays if no entries are found, and if no decoders are found vaapidecodebin is not registered. Also for vaapipostproc if no postproc entry is found.

Also, if general decoder, used by vaapidecodebin, doesn't have a sink pad string, don't register the glib type.

Fixes: #1349 (closed)