drm/i915: Implement WaSetupGtModeTdRowDispatch

I'm not really sure, since the w/a entry is as thin on details as
ever, and Bspec doesn't say anything about it. But I've figured only
dispatching to rows 0&1 instead of all four should be the right thing
for GT1.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
[danvet: Add the missing snb server GT1 to the check, spotted by Chris
Wilson.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>