pan/mdg: Enable nir_opt_algebraic_distribute_src_mods

pan/mdg: Enable nir_opt_algebraic_distribute_src_mods

Helps cleanup some issues otherwise missed by the new source mod
handling. (Noticed a double negative)

total instructions in shared programs: 3606 -> 3605 (-0.03%)
instructions in affected programs: 41 -> 40 (-2.44%)
helped: 1
HURT: 0

total bundles in shared programs: 1883 -> 1883 (0.00%)
bundles in affected programs: 0 -> 0
helped: 0
HURT: 0

total quadwords in shared programs: 3296 -> 3324 (0.85%)
quadwords in affected programs: 596 -> 624 (4.70%)
helped: 0
HURT: 2

total registers in shared programs: 337 -> 336 (-0.30%)
registers in affected programs: 6 -> 5 (-16.67%)
helped: 1
HURT: 0

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4852>