freedreno: fix issues with fp16 in preparation for mediump support

Right now the code to do precision lowering based on mediump/lowp information is a wip. But while working on it, we found some needed changes on the fp16 support.

Technically those are unnecessary until such lowering is finished, but they seem mature enough to be sent in advance while the lowering itself is finished. We think that it would be better to send them in advance, to leverage our development branch, and to receive feedback in advance.