We current have a single for_each_engine() iterator which we use to
generate both a set of uABI engines and a set of physical engines.
Determining what uABI ring-id corresponds to an actual HW engine is
tricky, so pull that out to a library function and introduce
for_each_physical_engine() for cases where we want to issue requests
once on each HW ring (avoiding aliasing issues).

v2: Remember can_store_dword for gem_sync
v3: Find more open-coded for_each_physical

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>