panfrost: Implement multiple render target support for fixed formats

Alyssa Rosenzweig requested to merge alyssa/mesa:mrt-v6 into master

This series fixes dEQP-GLES3.functional.fragment_out.array.fixed.*. It does not yet fix the non-fixed tests involving blend shaders, which have a bug relating to writeout (raising an INSTR_INVALID_PC fault) still, although this series does also get them closer.

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