Let edidDecodeFraction return float

Summary:
All uses of the function are QQuaternion setters which take floats.
This saves a bunch of conversions and should thus be faster. The edid
test shows that the result is unchanged in all digits.

Reviewers: romangg

Reviewed By: romangg

Subscribers: romangg, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D14368