nir: Stop whacking gl_FrontFacing to a system value

nir: Stop whacking gl_FrontFacing to a system value

We have a cap bit for gallium and a GLSL compiler flag to control this.
Just trust what GLSL gives us and stop forcing it.  In order for this to
be safe, we have to advertise another cap in some of the gallium
drivers.

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Eric Anholt <eric@anholt.net>