i965: Use unreachable() to silence warning.

brw_meta_fast_clear.c:211:17: warning: 'x_scaledown' may be used
uninitialized in this function [-Wmaybe-uninitialized]
    unsigned int x_scaledown, y_scaledown;

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

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

@@ -289,8 +289,7 @@ get_fast_clear_rect(struct brw_context *brw, struct gl_framebuffer *fb,
          x_scaledown = 2;
          break;
       default:
-         assert(!"Unexpected sample count for fast clear");
-         break;
+         unreachable("Unexpected sample count for fast clear");
       }
       y_scaledown = 2;
       x_align = x_scaledown * 2;