drm/panel: panel-dsi-cm: Use backlight helpers

Instead of retrieving the backlight brightness in struct
backlight_properties manually, and then checking whether the backlight
should be on at all, use backlight_get_brightness() which does all
this and insulates this from future changes.

Instead of setting the power state by manually updating fields in
struct backlight_properties, use backlight_enable() and
backlight_disable(). These also call backlight_update_status() so the
separate call is no longer needed.

Signed-off-by: Stephen Kitt <steve@sk2.org>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220616172316.1355133-3-steve@sk2.org