isl: Enable Unorm Path in Color Pipe

Improves performance on my Icelake 8x8 locked to 700Mhz. For example, some GfxBench5 subtests have the following results:

- [i965] gl_manhattan: ................ 7.01119% +/- 0.180971% (n=5)
- [i965] gl_4 (Car Chase):              4.24351% +/- 0.175622% (n=5)
- [i965] gl_blending:  ................ 3.36327% +/- 0.180267% (n=5)
- [i965] gl_5_normal (Aztec Ruins):     1.67962% +/- 0.243534% (n=10)
- [iris] gl_manhattan: ................ 3.92357% +/- 0.073965% (n=25)
- [iris] gl_4 (Car Chase):              2.17746% +/- 0.0826858% (n=5)
- [iris] gl_blending:  ................ 2.79599% +/- 0.803652% (n=15)
- [iris] gl_5_normal (Aztec Ruins):     1.30930% +/- 0.106523% (n=25)