mp_image: refine check for supported Dolby Vision metadata

Improve the Dolby Vision metadata check to accept RPUs that indicate EL
usage (profiles 4 and 7) if the original bitstream and metadata are
MEL-only, instead of hardcoding a check for disable_residual_flag.

This needs libplacebo commit a4783f89 ("util/libav: do not bail on
MEL-only RPUs w/EL"), or libplacebo API version 359.