do not convert signed RGTC via float

This is just something I noticed while working on !18248 (merged). It doesn't seem to make a functional difference.

@airlied: You might know more about the float-motivation here, as the code originated from you. I could imagine this was in order to not depend on SNORM support, but that seems a bit strange since this is SNORM functionality in the first place, so I wouldn't expect a driver to be able to support RGTC without already supporting SNORM. Anything I'm missing here?