drm/i915: disable CPU PWM also on LPT/SPT backlight disable

Although we don't support or enable CPU PWM with LPT/SPT based systems,
it may have been enabled prior to loading the driver. Disable the CPU
PWM on LPT/SPT backlight disable to avoid warnings on LCPLL disable.

The issue has been present on BDW since BDW enabling, but was recently
introduced on HSW with

commit 437b15b8
Author: Jani Nikula <jani.nikula@intel.com>
Date:   Fri Sep 4 16:55:13 2015 +0300

    drm/i915: use pch backlight override on hsw too

Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reference: http://mid.gmane.org/87y4frhwsn.fsf@yhuang-dev.intel.com


Reported-by: kernel test robot <ying.huang@intel.com>
Tested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1446033429-8006-1-git-send-email-jani.nikula@intel.com