glsl: fix glsl to nir support for lower precision builtins
When we switch to the full nir based glsl linker in an upcoming merge request this is required for existing tests from 8fcf8e7f to continue to pass, this is because they never exercised glsl to nir so the missing support went unnoticed.
Closes: #11456 (closed) Fixes: 8fcf8e7f ("glsl: lower builtins to mediump that ignore precision of certain parameters ")