virtio: vaapi: refuse formats that cannot decode to anything for now
virtio: vaapi: refuse formats that cannot decode to anything for now

The current code will add an input format even though it might not be
able to be decoded to NV12. This will be fixed in a later patch, but for
now just drop these formats instead.

BUG=b:214478588
TEST=cargo test -p devices --features=video-decoder,vaapi virtio::video::decoder::backend::vaapi::tests::test_get_capabilities -- --include-ignored

Change-Id: I89e7261fdc935207f1fcb11d18e7701cdaf54ba9
Reviewed-on: https://chromium-review.googlesource.com/c/crosvm/crosvm/+/3901274
Reviewed-by: Keiichi Watanabe <815e4e8f7c92a23424d7c3f51ead622bcedbc720@chromium.org>
Commit-Queue: Alexandre Courbot <301cd6a3ea50a894b4b51e385c5eb12b9a4ffc0c@chromium.org>
Reviewed-by: Alexandre Courbot <301cd6a3ea50a894b4b51e385c5eb12b9a4ffc0c@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm