r600: Correct evaluation of cube array index and face

r600: Correct evaluation of cube array index and face

The array index needs to be corrected and it must be insured that it is
rounded and its value is non-negative before it is combined with the
face id.

v5: Use RNDNE instead of ADD 0.5 and FLOOR (Ilia Mirkin)

v6: Fix type (Roland Scheidegger)

Fixes 182 from android/cts/master/gles31-master.txt:
  dEQP-GLES31.functional.texture.filtering.cube_array.formats.*
  dEQP-GLES31.functional.texture.filtering.cube_array.sizes.*
  dEQP-GLES31.functional.texture.filtering.cube_array.combinations.nearest_mipmap_*
  dEQP-GLES31.functional.texture.filtering.cube_array.combinations.linear_mipmap_*
  dEQP-GLES31.functional.texture.filtering.cube_array.no_edges_visible.*

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>