In sys.master_files there is a flag called is_percent_growth which is set to 1 if the file growth is in %

You can therefore work out the filegrowth as something like:

CASE WHEN f.is_percent_growth = 1

THEN CONVERT(VARCHAR(3), f.growth) + ' %'

ELSE CONVERT (VARCHAR(5), f.growth / 128) + ' MB'

END AS 'Growth'