mesa: fix bug in GPU codegen for fixed-function two-sided lighting

mesa: fix bug in GPU codegen for fixed-function two-sided lighting

The 'dots' register wasn't getting properly un-negated and un-swizzled
after emitting the code for back-face lighting.  So, if more than one
light source was enabled, the specular exponent for the next light source
was wrong.

During execution we were evaluating pow(x, y) where y was negative instead
of positive.  This led to the outcome being zero or NaN.

This fixes the occasional black triangles seen in isosurf when hacked to
enable two-sided lighting.