i965: Fix glFrontFacing in twoside GLSL demo.

i965: Fix glFrontFacing in twoside GLSL demo.

This also cuts instructions by just using the existing bit in the payload
rather than computing it from the determinant in the SF unit and passing it
as a varying down to the WM.  Something still goes wrong with getting the
backface color right, but a simpler shader appears to get the right result.