utils/libav: allow user control of HAVE_LAV_VULKAN

[quink-black]

Check libavutil version and PL_HAVE_VULKAN is necessary but not sufficient. libavutil can missing vulkan support, which leads to link error like:

Undefined symbols for architecture x86_64:
  "_av_vkfmt_from_pixfmt", referenced from:
      _map_frame in plplay.c.o

The patch only fix libplacebo as a library. We needs to check whether av_vkfmt_from_pixfmt is available and fix the link error of demo.

[haasn]

I still think this is a bug in FFmpeg that needs to be addressed there. I'll submit a patch there, I think.

[haasn]

Fixed upstream