SpacemiT K1's BSP has a version of FFmpeg that contains both
v4l2m2m and their own *_stcodec decoders. Not only is their
V4L2 driver for their mvx decoder seemingly totally broken
(output buffers are all zeros), but it doesn't have any of
the DRM_PRIME support work that is maintained out of tree.
The result is we use a broken copyback decoder instead of
a (mostly) working zero-copy decoder because we didn't
include any logic to rank non-hwaccel decoders by pixfmt
like we have with device type for hwaccel decoders.
952ebcd0d2
