panfrost: Only write depth / stencil once if MRT is used

We can't assume that RT0 will be written, so this has to be based on
whether a combined store has already been emitted, not the location of
the store.

Emit a non-special combined_store intrinsic that only writes colour
for the other RTs, as reordering stores breaks the Midgard compiler.

Fixes: d37e901e ("pan/mdg: Add new depth store lowering")
Closes: #6527
Part-of: <!16685>
174 jobs for !16685 with mrt-fragdepth-fix in 45 minutes and 36 seconds (queued for 11 seconds)
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