drm/i915/cnl: Drop all workarounds

All of the Cannon Lake hardware that came out had graphics fused off,
and our userspace drivers have already dropped their support for the
platform; CNL-specific code in i915 that isn't inherited by subsequent
platforms is effectively dead code.  Let's remove all of the
CNL-specific workarounds as a quick and easy first step.

References: https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6899


Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210713193635.3390052-12-matthew.d.roper@intel.com