intel/genxml: Simplify prefix handling for field value lists

intel/genxml: Simplify prefix handling for field value lists

When a <field> tag has multiple <value> children, listing symbolic names
for possible field values, we generate #defines for each value, with an
optional prefix.  I don't know why, but this code was checking whether
self.default is None.  We want to generate the same list of #defines,
with a prefix, regardless of whether the field has a default value
specified or not.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13809>