intel/decoder: Use 'DWord Length' and 'bias' fields for packet length.

intel/decoder: Use 'DWord Length' and 'bias' fields for packet length.

Use the 'DWord Length' and 'bias' fields from the instruction definition to
parse the packet length from the command stream when possible. The hardcoded
mechanism is used whenever an instruction doesn't have this field.

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