drm/i915: Use intel_panel_actually_set_backlight() to disable PWM backlight

drm/i915: Use intel_panel_actually_set_backlight() to disable PWM backlight

Use intel_panel_actually_set_backlight() instead of a direct
call to pwm_config() in pwm_disable_backlight().

The main benefit is consistent debug logging when we turn off the
backlight. Currently we see nothing in dmesg which made me wonder
whether the backlight was even getting turned off properly.

The second benefit is consistency; This is what we do for all
the other backlight implementations.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181024155208.18987-1-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Jani Nikula <jani.nikula@intel.com>