panfrost/ci: Split rules by ISA
Panfrost has two compilers, one for Midgard GPUs and one for Bifrost GPUs. The respective compilers are src/panfrost/midgard and src/panfrost/bifrost. Changes internal to just one compiler (or disassembler) cannot affect the other hardware, so there's no need to run extra jobs in these cases.
Cc @tomeu