I have the exact same problem as you do! The difference is you've got it for years already, while I only have it quite recently. So yes, I left a scar on the shaft of my penis after masturbating last month, and it was quite bad actually, it looked like a couple of blisters. I applied povidone iodine and Garamycin on the wound, and it healed completely after about 2 weeks.
The thing is, I now have this white spot similar to what you described precisely on the same spot where the wound existed. The spot looks quite obvious and annoying since I'm also Asian so my penis is dark. I tried asking around and browsing on the net, and I found this cream called Dermatix. They say it's effective in healing skin discoloration and scars resulting from healed wounds. But I frankly don't know about scars which have been there for 6 years, I suppose the early you apply it the better the result is...
Since the discoloration has been present for quite some time, it can be hypopigmentation or vitiligo skin. Vitiligo is a pigmentation disorder in which melanocytes (the cells that make pigment) in the skin are destroyed. As a result, white patches appear and for confirming the diagnosis, biopsy needs to be done.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.