drm/i915: Apply headless DMC workaround for CNL

With firmware 1.07 having fixed the state corruption issue, we can enable
the headless GT performance workaround for CNL as well. (Equivalent to
b6876374 ("drm/i915: Restore GT performance in headless mode with DMC
loaded") on other affected platforms.)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=100572


Testcase: igt/gem_exec_nop/headless
Cc: Imre Deak <imre.deak@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Dmitry Rogozhkin <dmitry.v.rogozhkin@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180111082417.795-1-tvrtko.ursulin@linux.intel.com