i965: Disable channel expressions for scalar GS, TCS, TES.

i965: Disable channel expressions for scalar GS, TCS, TES.

On Broadwell, I get the following shader-db statistics:

Tessellation Control Shaders:

   total instructions in shared programs: 57327 -> 57012 (-0.55%)
   instructions in affected programs: 27334 -> 27019 (-1.15%)
   helped: 45
   HURT: 0

   total cycles in shared programs: 265692 -> 255188 (-3.95%)
   cycles in affected programs: 263122 -> 252618 (-3.99%)
   helped: 184
   HURT: 26

Tessellation Evaluation Shaders:

   total instructions in shared programs: 23236 -> 23157 (-0.34%)
   instructions in affected programs: 2791 -> 2712 (-2.83%)
   helped: 27
   HURT: 0

   total cycles in shared programs: 151858 -> 149704 (-1.42%)
   cycles in affected programs: 151858 -> 149704 (-1.42%)
   helped: 101
   HURT: 114

Geometry Shaders:

   Orbital Explorer goes from 6442 -> 6356 instructions.
   Two Shadow of Mordor shaders increase by a single instruction.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>