genxml: Fix decoder for groups with multiple fields.

genxml: Fix decoder for groups with multiple fields.

If you have something like:

    <group count="0" start="96" size="32">
      <field name="Entry_0" start="0" end="15" type="GATHER_CONSTANT_ENTRY"/>
      <field name="Entry_1" start="16" end="31" type="GATHER_CONSTANT_ENTRY"/>
    </group>

We would reset ctx->group_count to 0 after processing the first field,
so the second would not have a group count.

This is largely untested, as the only groups with multiple fields are
packets we don't emit in Mesa.  Found by inspection.

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