drm/i915/cnp: Wa 1181: Fix Backlight issue

This workaround fixes a CNL PCH bug when changing
backlight from a lower frequency to a higher frequency.

During random reboot cycles, display backlight seems to
be off/ dim for 2-3 mins.

The only functional change on this patch is to
set bit 13 of 0xC2020 for CNL PCH.

The rest of patch is organizing identation around
those bits definitions and re-organizing CFL workarounds.

v2: Only add the bit that matters without touching others
    around (Jani).
    Rebase on top of clock gating functions rename.

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Arthur J Runyan <arthur.j.runyan@intel.com>
Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170831045223.3960-1-rodrigo.vivi@intel.com