intel/isl: Fix packing of SINT formats

Prevents the next patch from failing many multisampled, signed integer
rendering tests. For example:

 dEQP-VK.renderpass2.suballocation.multisample_resolve.r8_sint.samples_4

Cc: mesa-stable
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/30646>

src/intel/isl/isl_format.c

@@ -1294,7 +1294,7 @@ pack_channel(const union isl_color_value *value, unsigned i,
       packed = MIN(value->u32[i], u_uintN_max(layout->bits));
       break;
    case ISL_SINT:
-      packed = CLAMP(value->u32[i], u_intN_min(layout->bits),
+      packed = CLAMP(value->i32[i], u_intN_min(layout->bits),
                      u_intN_max(layout->bits));
       break;