freedreno/ir3: Don't use isam for coherent image loads on a5xx+

If the coherent flag is present, then we need to not have an incoherent cache between us and previous stores to the image that were also decorated as coherent. isam apparently (unsurprisingly) goes through a texture cache. Use ldib instead, so that we don't get the wrong result.

We need a similar fix for a4xx, but we don't have emit_intrinsic_load_image for those (yet).