mesa: Avoid using the shine table for _tnl_RasterPos

mesa: Avoid using the shine table for _tnl_RasterPos

Use direct computation of pow for computing the shininess
in _tnl_RasterPos. Since the _tnl_RasterPos function is still
used by plenty drivers that do only need the shine table for
_tnl_RasterPos but do not make use of swtnl computations, this
enables pushing down the shine table computation and validation
into the tnl module, which will happen in a followup change.

Reviewed-by: Alex Deucher <alexdeucher@gmail.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Signed-off-by: Mathias Froehlich <Mathias.Froehlich@web.de>