intel/genxml: Fix decoding of groups with fields smaller than a DWord.

intel/genxml: Fix decoding of groups with fields smaller than a DWord.

Groups containing fields smaller than a DWord were not being decoded
correctly.  For example:

    <group count="32" start="32" size="4">
      <field name="Vertex Element Enables" start="0" end="3" type="uint"/>
    </group>

gen_field_iterator_next would properly walk over each element of the
array, incrementing group_iter, and calling iter_group_offset_bits()
to advance to the proper DWord.  However, the code to print the actual
values only considered iter->field->start/end, which are 0 and 3 in the
above example.  So it would always fetch bits 3:0 of the current DWord
when printing values, instead of advancing to each element of the array,
printing bits 0-3, 4-7, 8-11, and so on.

To fix this, we add new iter->start/end tracking, which properly
advances for each instance of a group's field.

Caught by Matt Turner while working on 3DSTATE_VF_COMPONENT_PACKING,
with a patch to convert it to use an array of bitfields (the example
above).

This also fixes the decoding of 3DSTATE_SBE's "Attribute Active
Component Format" fields.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>