Disallow NaN in cellVideoOutSetGamma

[elad335]

This matches the firmware as it errors if gamma NGE 0.8 (Not Greater Equal Than).

[kd-11]

While this disallows nan, its obviously by accident and makes the code less readable for no gain (nobody is going to call setgamma with NaN). As a concession, just invert the whole condition whether it matches firmware or not. I still think its not actually useful though.

[elad335]

Done, can also use std::isnan for readability but it currently seem redundent.

[elad335]

Also fixed error checking order.