encoder: fix and factor out check for supported rate-control modes.

encoder: fix and factor out check for supported rate-control modes.

Cache the first compatible GstVaapiProfile found if the encoder is not
configured yet. Next, factor out the code to check for the supported
rate-control modes by moving out vaGetConfigAttributes() to a separate
function, while also making sure that the attribute type is actually
supported by the encoder.

Also fix the default set of supported rate control modes to not the
"none" variant. It's totally useless to expose it at this point.