anv/skylake: disable ForceThreadDispatchEnable

anv/skylake: disable ForceThreadDispatchEnable

On Skylake enabling of ForceThreadDispatchEnable causes gpu-hang.

-v2: enabling of  ForceThreadDispatchEnable is only for gen8, for
     gen9 and higher reverted enabling of PixelShaderHasUAV.

-v3 (Jason Ekstrand): Rework the comments a bit.

CC: Jason Ekstrand <jason.ekstrand@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107941
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107760
Fixes: 79270d2140ec (anv: Stop setting 3DSTATE_PS_EXTRA::PixelShaderHasUAV)
Signed-off-by: Sergii Romantsov <sergii.romantsov@globallogic.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>