i965: Add an assert for when SET_FIELD's value exceeds the field size.

i965: Add an assert for when SET_FIELD's value exceeds the field size.

This was one of the things we always wanted to do to this, to make it more
useful than just (value << FIELD_MASK).

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>