PLEASE READ
The 'grainy' appearance is a FEATURE not a BUG . It is useful in seeing the phone in direct sunlight.
If you dont want this FEATURE, you can TURN IT OFF IF you have portico
Go to settings, display+touch and turn off sun readability.
I tried that and it still happens. i've compared my phone with another 920 with both settings with sunlight readability on and off and my phone with the issue i'm describing still occurs. you don't know what i'm talking about because obviously you don't have the bug. my phone goes so bright it's actually now unreadable. this has gotten worse since i bought the phone.
i'll be able to confirm if Nokia thinks this is a bug because i just sent it for warranty repairs and i'll see what they say in reply.