I don't think you can once they have gone like that, as the ammonia in the urine has kind of soaked in and contaminated the area. I tried using the pressure washer on those areas last year and it did not 'remove' it
if they are solid rubber mats it is highly likely that the urine has begun to perish the mats so it is pretty diffiuclt to clean them up well. If they are the eva mats then they are actually dyed black so again the urine bleaches the colour out of them so can't do anything about it either!!
I have eva mats and just use flash on them when they get scubbed - really easy job as they are so light!! The solid rubber ones are just not so easy.....
Fieldguard do a 'Green Gloop' It smells lovely and brings you mats up like new, not sure it would completely remove the white stain but it is very very good. I think you can only buy it direct from them.