freedreno/ir3/a6xx: use ldib for ssbo reads

freedreno/ir3/a6xx: use ldib for ssbo reads

... instead of isam.  It seems like when using isam, plus atomics, we
can have the problem of old data being in the texture cache.  Plus this
way we don't have to load a component at a time.

Note that blob still seems to use isam in some cases.  I suppose it might
be preferable in the case of loading a single component, when atomics
are not in the picture (or that the ssbo does not need to otherwise be
coherent).

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