panfrost: UBO fixes for Bifrost

Boris Brezillon requested to merge bbrezillon/mesa:bifrost-ubo-fixes into master

Those 2 patches were previously part of !7744 (closed). As discussed with @alyssa, the second patch is a hack which should go away with !8354 (merged), but it allows us to get a bunch of GLES3 passing in the meantime.

/cc @alyssa

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