drm/i915: Move for_each_engine* out of i915_drv.h

Iterators operate on struct intel_gt so lets move it to intel_gt.h in
order to make i915_drv.h less of a dumping ground for stuff.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Suggested-by: Jani Nikula <jani.nikula@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231102093248.362659-2-tvrtko.ursulin@linux.intel.com