intel/decoder: Clean up field iteration and fix sub-dword fields

First of all, setting iter->name in advance_field is unnecessary because
it gets set by gen_decode_field which gets called immediately after
gen_decode_field in the one call-site.  Second, we weren't properly
initializing start_bit and end_bit in the initial condition of
gen_field_iterator_next so the first field of a struct would get printed
wrong if it doesn't start on the first bit.  This is fixed by adding a
iter_start_field helper which sets the field and also sets up the other
bits we need.  This fixes decoding of 3DSTATE_SBE_SWIZ.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
(cherry picked from commit 2abd7ae1)