i965/blorp: Mark branch unreachable to silence uninitialized var warning.

Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/mesa/drivers/dri/i965/brw_blorp_clear.cpp

@@ -340,6 +340,7 @@ brw_blorp_clear_params::brw_blorp_clear_params(struct brw_context *brw,
             break;
          default:
             assert(!"Unexpected sample count for fast clear");
+            unreachable();
             break;
          }
          y_scaledown = 2;