pan/mdg: Schedule writeout to VLUT

pan/mdg: Schedule writeout to VLUT

Many thanks to Icecream95 for noticing this is possible if alpha is not
written.

total instructions in shared programs: 50509 -> 50508 (<.01%)
instructions in affected programs: 221 -> 220 (-0.45%)
helped: 2
HURT: 1
helped stats (abs) min: 1 max: 1 x̄: 1.00 x̃: 1
helped stats (rel) min: 0.74% max: 1.35% x̄: 1.04% x̃: 1.04%
HURT stats (abs)   min: 1 max: 1 x̄: 1.00 x̃: 1
HURT stats (rel)   min: 9.09% max: 9.09% x̄: 9.09% x̃: 9.09%

total bundles in shared programs: 25675 -> 25640 (-0.14%)
bundles in affected programs: 5434 -> 5399 (-0.64%)
helped: 34
HURT: 0
helped stats (abs) min: 1 max: 2 x̄: 1.03 x̃: 1
helped stats (rel) min: 0.27% max: 20.00% x̄: 2.29% x̃: 0.67%
95% mean confidence interval for bundles value: -1.09 -0.97
95% mean confidence interval for bundles %-change: -3.64% -0.94%
Bundles are helped.

total quadwords in shared programs: 40887 -> 40899 (0.03%)
quadwords in affected programs: 1995 -> 2007 (0.60%)
helped: 2
HURT: 16
helped stats (abs) min: 1 max: 3 x̄: 2.00 x̃: 2
helped stats (rel) min: 1.67% max: 2.40% x̄: 2.03% x̃: 2.03%
HURT stats (abs)   min: 1 max: 1 x̄: 1.00 x̃: 1
HURT stats (rel)   min: 0.54% max: 5.88% x̄: 1.40% x̃: 0.86%
95% mean confidence interval for quadwords value: 0.15 1.18
95% mean confidence interval for quadwords %-change: 0.13% 1.90%
Quadwords are HURT.

total registers in shared programs: 3916 -> 3917 (0.03%)
registers in affected programs: 2 -> 3 (50.00%)
helped: 0
HURT: 1

total threads in shared programs: 2455 -> 2455 (0.00%)
threads in affected programs: 0 -> 0
helped: 0
HURT: 0

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5513>