Prevent TLWidthWidget from generating X outputs (#2815)

* Prevent TLWidthWidget from generating X outputs

Widening WidthWidgets contain an uninitialized data register.  The first
transaction through the widget may propagate that register's contents
on the bus, which is logically correct but can propagates a X, violating
our constraint that we never provide X in the payload of a valid xact.

Fix by propagating in.bits.data, rather than the register, until the
register has been written at least once.  This is cheaper than resetting
the register.

* Add comments to WidthWidget X fix

Co-authored-by: Andrew Waterman <andrew@sifive.com>