freedreno/ir3: handle quirky atomic dst for a6xx

freedreno/ir3: handle quirky atomic dst for a6xx

The new encoding returns a value via the 2nd src.  The legalize pass
needs to be aware of this to set the correct needs_sy flag, otherwise we
can, in cases where the atomic dst is not used, overwrite the register
that hardware will asynchronously load result into without (sy) flag, so
it gets clobbered by the atomic result.

This fixes a whole lot of rando ssbo+atomic fails, like
dEQP-GLES31.functional.ssbo.layout.single_basic_type.packed.highp_vec4.

Signed-off-by: Rob Clark <robdclark@gmail.com>