i965/fs: Fix linear gl_Color interpolation on pre-gen6 hardware.
i965/fs: Fix linear gl_Color interpolation on pre-gen6 hardware.

Civilization 4's shaders make heavy use of gl_Color and don't use
perspective interpolation.  This resulted in rivers, units, trees, and
so on being rendered almost entirely white.  This is a regression
compared to the old fragment shader backend.

Found by inspection (comparing the old and new FS backend code).

References: https://bugs.freedesktop.org/show_bug.cgi?id=32949

NOTE: This is a candidate for the 7.10 branch.

bkaradzic/glsl-optimizer,zz85/glsl-optimizer,zeux/glsl-optimizer,jbarczak/glsl-optimizer,benaadams/glsl-optimizer,wolf96/glsl-optimizer,benaadams/glsl-optimizer,mapbox/glsl-optimizer,mcanthony/glsl-optimizer,bkaradzic/glsl-optimizer,zz85/glsl-optimizer,KTXSoftware/glsl2agal,dellis1972/glsl-optimizer,tokyovigilante/glsl-optimizer,mcanthony/glsl-optimizer,benaadams/glsl-optimizer,mapbox/glsl-optimizer,adobe/glsl2agal,zeux/glsl-optimizer,KTXSoftware/glsl2agal,adobe/glsl2agal,benaadams/glsl-optimizer,zz85/glsl-optimizer,KTXSoftware/glsl2agal,zz85/glsl-optimizer,zz85/glsl-optimizer,wolf96/glsl-optimizer,wolf96/glsl-optimizer,tokyovigilante/glsl-optimizer,zz85/glsl-optimizer,metora/MesaGLSLCompiler,mapbox/glsl-optimizer,djreep81/glsl-optimizer,KTXSoftware/glsl2agal,djreep81/glsl-optimizer,mcanthony/glsl-optimizer,dellis1972/glsl-optimizer,adobe/glsl2agal,bkaradzic/glsl-optimizer,djreep81/glsl-optimizer,adobe/glsl2agal,benaadams/glsl-optimizer,tokyovigilante/glsl-optimizer,dellis1972/glsl-optimizer,zeux/glsl-optimizer,jbarczak/glsl-optimizer,wolf96/glsl-optimizer,metora/MesaGLSLCompiler,djreep81/glsl-optimizer,adobe/glsl2agal,djreep81/glsl-optimizer,jbarczak/glsl-optimizer,wolf96/glsl-optimizer,metora/MesaGLSLCompiler,jbarczak/glsl-optimizer,mapbox/glsl-optimizer,tokyovigilante/glsl-optimizer,KTXSoftware/glsl2agal,tokyovigilante/glsl-optimizer,mcanthony/glsl-optimizer,bkaradzic/glsl-optimizer,jbarczak/glsl-optimizer,zeux/glsl-optimizer,mapbox/glsl-optimizer,dellis1972/glsl-optimizer,benaadams/glsl-optimizer,bkaradzic/glsl-optimizer,zeux/glsl-optimizer,mcanthony/glsl-optimizer,dellis1972/glsl-optimizer