i965/simd8vs: Fix SIMD8 atomics (read-only)

i965/simd8vs: Fix SIMD8 atomics (read-only)

An update for d9cd982d556be560af3bcbcdaf62b6b93eb934a5.

A similar change was needed for CS to allow the piglit test
tests/spec/arb_compute_shader/execution/simple-barrier-atomics.shader_test
to pass.

The previous change (d9cd982d) should fix cases that write atomics,
such as atomicCounterIncrement, and this change will fix cases than
only read atomics, such as atomicCounter.

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>