ffmpeg: Don't require a known device to pass a frames context to an encoder

ffmpeg: Don't require a known device to pass a frames context to an encoder

The previous code here did not handle passing a frames context when
ffmpeg itself did not know about the device it came from (for example,
because it was created by device derivation inside a filter graph), which
would break encoders requiring that input.  Fix that by checking for HW
frames and device context methods independently, and prefer to use a
frames context method if possible.  At the same time, revert the encoding
additions to the device matching function because the additional
complexity was not relevant to decoding.

Also fixes #8637, which is the same case but with the device creation
hidden in the ad-hoc libmfx setup code.