intel/genxml: fix bits generation for MI_LOAD_REGISTER_IMM

intel/genxml: fix bits generation for MI_LOAD_REGISTER_IMM

This instruction has a group with the same name than another field above :

  <field name="Data DWord" start="64" end="95" type="uint"/>
  <group count="0" start="96" size="64">
    <field name="Register Offset" start="2" end="22" type="offset"/>
    <field name="Data DWord" start="32" end="63" type="uint"/>
  </group>

The script was replacing the offset of the field first with the second
one in the group.

This change ignore anything a group within an instruction.

v2: Drop unused variable (Rafael)

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Rafael Antognolli <rafael.antognolli@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/2775>