pan/mdg: eliminate references to ins->alu.outmod

pan/mdg: eliminate references to ins->alu.outmod

In an effort to simplify MIR by not prepacking instructions, this commit
removes references to `ins->alu.outmod` so that we can later remove the
`ins->alu` field from midgard_instruction.
Every place that was using `ins->alu.outmod` was changed to now use the
generic `ins->outmod` field instead.
We then reconstruct the outmod field right before emission.

Signed-off-by: Italo Nicola <italonicola@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5933>