drm/i915/gem: Mark up sw-fence notify function

The sw-fence notify function requires to be at least 4-byte aligned so
that we can use the low bits in the function pointer for internal fence
flags. Make it so.

References: https://gitlab.freedesktop.org/drm/intel/issues/1433
Fixes: 42fb60de

 ("drm/i915/gem: Don't leak non-persistent requests on changing engines")
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>
Link: https://patchwork.freedesktop.org/patch/msgid/20200311221739.30375-1-chris@chris-wilson.co.uk