radv/ac: add support for multi sample image coords

radv/ac: add support for multi sample image coords

This just adds the nir->llvm support, enabling
the extension causes some failures on llvm 3.9 at least,
but this code seems fine.

NIR passes the sampler in src[1].x, and we LLVM/SI requires
it as the last parameters in the coords (coord[2] for 2D,
coord[3] for 2DArray).

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Signed-off-by: Dave Airlie <airlied@redhat.com>