tgsi: simplify opcode declaration

Gert Wollny requested to merge gerddie/virglrenderer:tgsi-fix-opcode into main

The opcode IDs are not in sync with mesa, and we actually never need specific opcode IDs, because we translate the shaders from text. With that opcode TGSI_OPCODE_U642D becomes valid again (with a value of 256 it was out of range for the eight bit value in tgsi_instruction.

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