radeonsi: Fix crash on shaders using MSAA image load/store

radeonsi: Fix crash on shaders using MSAA image load/store

The value returned by tgsi_util_get_texture_coord_dim() does not
account for the sample index. This means image_fetch_coords() will not
fetch it, leading to a null deref in ac_build_image_opcode() which
expects it to be present (the return value of ac_num_coords() *does*
include the sample index).

Signed-off-by: Alex Smith <asmith@feralinteractive.com>
Cc: "18.1" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>