drm/i915/backlight: Make ext_pwm_disable_backlight() call intel_backlight_set_pwm_level()

At least the Bay Trail LPSS PWM controller used with DSI panels on many
Bay Trail tablets seems to leave the PWM pin in whatever state it was
(high or low) ATM that the PWM gets disabled. Combined with some panels
not having a separate backlight-enable pin this leads to the backlight
sometimes staying on while it should not (when the pin was high during
PWM-disabling).

First calling intel_backlight_set_pwm_level() will ensure that the pin
is always low (or high for inverted brightness panels) since the passed
in duty-cycle is 0% (or 100%) when the PWM gets disabled fixing the
backlight sometimes staying on.

With the exception of ext_pwm_disable_backlight() all other
foo_disable_backlight() functions call intel_backlight_set_pwm_level()
already before disabling the backlight, so this change also aligns
ext_pwm_disable_backlight() with all the other disable() functions.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211121110032.4720-2-hdegoede@redhat.com