mesa/st and freedreno: Remap VARYING_SLOT_PNTC properly
freedreno seems to be the only Gallium driver using NIR not to enable
PIPE_CAP_TGSI_TEXCOORD. The state tracker is supposed to map the special TEXCOORD varyings as well as
gl_PointCoord to normal varyings in this case, but it was failing to do so for
gl_PointCoord which led to a hack in ir3 that was breaking turnip.
This is one of two fixes required for
gl_PointCoord to work correctly in turnip, but I thought I'd split it out for visibility as it touches mesa/st.