i915g: Turn off FP16 in the vertex shaders.
This ended up being turned on in gallivm, but since we use nir_to_tgsi on the VS and TGSI doesn't have FP16, we can't let that happen. Fixes: f814a244 ("llvmpipe: enable FP16 and update CL + traces piglit results.") Part-of: <mesa/mesa!14403> (cherry picked from commit b9e8936b)