drm/i915: Replace the line of %s by a DEV_INFO_FOR_EACH_FLAG() invocation

This way, when adding a device flag we don't have to manually maintain
that list.

v2: undefine the helper macros (Jani Nikula, Daniel Vetter)

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>