llvmpipe: say no to depth clamp

The other drivers just return 0 without the assert.

src/gallium/drivers/llvmpipe/lp_screen.c

@@ -169,6 +169,8 @@ llvmpipe_get_param(struct pipe_screen *screen, enum pipe_cap param)
       return 1;
    case PIPE_CAP_GEOMETRY_SHADER4:
       return 1;
+   case PIPE_CAP_DEPTH_CLAMP:
+      return 0;
    default:
       assert(0);
       return 0;