drm/i915/cnl: Apply large line width optimization

This bit enables hardware that will change the approximation used for distances
calculations for AA wide lines so that they are rendered more accurately.

The default value for this bit leaves the legacy behavior. There is no good
reason to not enable the new approximation except if comparing to previous GEN
rendered images.

v2: Rebase
v3: Fix author.
    Rebased by Rodrigo who also added  a comment as suggested by Oscar.
    Since it is surrounded by Workarounds let's just add a comment to
    make clear it is not an Wa.

Cc: Oscar Mateo <oscar.mateo@intel.com>
Signed-off-by: Ben Widawsky <benjamin.widawsky@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Oscar Mateo <oscar.mateo@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170815231651.975-4-rodrigo.vivi@intel.com