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.