genxml: Fix decoding of array groups.

genxml: Fix decoding of array groups.

If you had a group as the first element of a struct, i.e.

  <struct name="3DSTATE_CONSTANT_BODY" length="10">
    <group count="4" start="0" size="16">
      <field name="ReadLength" start="0" end="15" type="uint"/>
    </group>
    ...
  </struct>

we would get a group_offset of 0, causing create_field() to think the
field wasn't in a group, and fail to offset forward for successive array
elements.  So we'd mark all the array elements as offset 0.

Using ctx->group->elem_size is a better check for "are we in a group?".

Acked-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>