i965: Prevent coverity warning

Add assert checking that num_sources is never larger than 3.

This prevents Coverity from concluding that the unhandled
cases of num_sources not being 0-3 are relevant.

Coverity-Id: 1399480-1399489
Signed-off-by: Robert Foss <robert.foss@collabora.com>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>

src/mesa/drivers/dri/i965/brw_eu_validate.c

@@ -177,6 +177,7 @@ num_sources_from_inst(const struct gen_device_info *devinfo,
          return 0;
       }
    } else {
+      assert(desc->nsrc < 4);
       return desc->nsrc;
    }