Now that host structures are indexed by host engine-id rather than
guc_id, we can usefully convert some for_each_engine() loops to use
for_each_engine_id() and avoid multiple dereferences of engine->id.

Also a few related tweaks to cache structure members locally wherever
they're used more than once or twice, hopefully eliminating memory
references.
Signed-off-by: NDave Gordon <david.s.gordon@intel.com>
Reviewed-by: NTvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: NTvrtko Ursulin <tvrtko.ursulin@intel.com>