plugins: fix check for direct-rendering support.

plugins: fix check for direct-rendering support.

Fix check for direct-rendering if the creation of VA surfaces with
an explicit pixel format is not support, e.g. VA-API < 0.34.0, and
that we tried to allocate a VA surface based on the corresponding
chroma type. i.e. in that particular case, we have to make sure that
the derived image has actually the expected format.