Your machine should work with hw vulkan driver for AMD GPU, but there is method to run same filter with software "lavapipe" (much slower, of course)

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Environment vars: LIBGL_ALWAYS_SOFTWARE=1 __GLX_VENDOR_LIBRARY_NAME=mesa VK_ICD_FILENAMES=/usr/share/vulkan/icd.d/lvp_icd.i686.json:/usr/share/vulkan/icd.d/lvp_icd.x86_64.json

Command: ffmpeg -i video.mkv -init_hw_device vulkan -vf hwupload,libplacebo=format=yuv420p,hwdownload,format=yuv420p test.mkv       

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problem with self-building libplacebo it build-depends on some quite heavy Vulkan shader compiler (?)  thing, termux does have libplacebo packaged for mpv but binary package for ffmpeg (7.1.1) was built without vulkan support ...