avcodec/libvpxdec: don't check for formats other than i420 when vp9 decoding is disabled

Should fix ticket #3958.

Reviewed-by: James Zern <jzern@google.com>
Signed-off-by: James Almer <jamrial@gmail.com>
x86/videodsp: fix warning about discarded 'const' qualifier

Reviewed-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: James Almer <jamrial@gmail.com>
hevc: remove unneeded buffer.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
hevc: move intermediate bidir buffer

Other buffers are already there.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>