pan/mdg: fix midgard writemask encoding for stores
This commits fixes a regression caused by commit 616394cf.
Midgard's non-image and non-varying store operations have a unique behavior for the load/store writemask, with each bit being responsible for 1/4th of the size of the total write operation. This means that we have to pack the writemask differently since we stopped using st_u128 for everything.
Fixes:
- dEQP-GLES31.functional.compute.shared_var.atomic.min.*,
- dEQP-GLES31.functional.ssbo.atomic.min.*,
- dEQP-GLES31.functional.compute.shared_var.atomic.max.* and
- dEQP-GLES31.functional.ssbo.atomic.max.*.
And possibly others, I haven't tested with a full gles31 run.