intel: decoder: identify groups with fixed length

intel: decoder: identify groups with fixed length

<register> & <struct> elements always have fixed length. The
get_length() method implies that we're dealing with an instruction in
which the length is encoded into the variable data but the field
iterator uses it without checking what kind of gen_group it is dealing
with.

Let's make get_length() report the correct length regardless of the
gen_group (register, struct or instruction).

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>