panfrost: Unit test pan_blend.c and wire in support for 2*src*dst blending on BIfrost

Merged Alyssa Rosenzweig requested to merge alyssa/mesa:blend-tests into main

Cc @bbrezillon

I wrote this in the car on my midgard laptop, so it's not tested except for the unit tests. Running through CI now and then signing off for the weekend. But hey.

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