Provide the iterator name as an explicit macro parameter so that it is
known to the caller, and allows for them to properly nest loops over all
engines.

Fixes:
../tests/i915/gem_exec_schedule.c: In function ‘semaphore_noskip’:
../lib/igt_gt.h:84:44: warning: declaration of ‘e__’ shadows a previous local [-Wshadow]
  for (const struct intel_execution_engine *e__ = intel_execution_engines;\
                                            ^~~
../tests/i915/gem_exec_schedule.c:653:2: note: in expansion of macro ‘for_each_physical_engine’
  for_each_physical_engine(i915, other) {
  ^~~~~~~~~~~~~~~~~~~~~~~~
../lib/igt_gt.h:84:44: note: shadowed declaration is here
  for (const struct intel_execution_engine *e__ = intel_execution_engines;\
                                            ^~~
../tests/i915/gem_exec_schedule.c:652:2: note: in expansion of macro ‘for_each_physical_engine’
  for_each_physical_engine(i915, engine) {
  ^~~~~~~~~~~~~~~~~~~~~~~~
../tests/i915/gem_exec_schedule.c: In function ‘measure_semaphore_power’:
../lib/igt_gt.h:84:44: warning: declaration of ‘e__’ shadows a previous local [-Wshadow]
  for (const struct intel_execution_engine *e__ = intel_execution_engines;\
                                            ^~~
../tests/i915/gem_exec_schedule.c:1740:3: note: in expansion of macro ‘for_each_physical_engine’
   for_each_physical_engine(i915, engine) {
   ^~~~~~~~~~~~~~~~~~~~~~~~
../lib/igt_gt.h:84:44: note: shadowed declaration is here
  for (const struct intel_execution_engine *e__ = intel_execution_engines;\
                                            ^~~
../tests/i915/gem_exec_schedule.c:1719:2: note: in expansion of macro ‘for_each_physical_engine’
  for_each_physical_engine(i915, signaler) {
  ^~~~~~~~~~~~~~~~~~~~~~~~

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