vaapi_encode: Choose profiles dynamically

Previously there was one fixed choice for each codec (e.g. H.265 -> Main
profile), and using anything else then required an explicit option from
the user.  This changes to selecting the profile based on the input format
and the set of profiles actually supported by the driver (e.g. P010 input
will choose Main 10 profile for H.265 if the driver supports it).

The entrypoint and render target format are also chosen dynamically in the
same way, removing those explicit selections from the per-codec code.