panfrost: Enable NIR lowering of half float packing

The GLSL lowering of half float packing involves software conversion
to half-float; instead, use the lowering in NIR.

Both Midgard and Bifrost are already set to lower the instructions to
bit operations, but change mdg_should_scalarize so that the lowerable
split variants of the pack/unpack instructions are generated.

Part-of: <!16175>
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