drm/panel: add backlight support

Panels often support backlight as specified in a device tree.
Update the drm_panel infrastructure to support this to
simplify the drivers.

With this the panel driver just needs to add the following to the
probe() function:

    err = drm_panel_of_backlight(panel);
    if (err)
            return err;

Then drm_panel will handle all the rest.

There is one caveat with the backlight support.
If drm_panel_(enable|disable) are called multiple times
in a row then backlight_(enable|disable) will be called multiple times.

The above will happen when a panel drivers unconditionally
calls drm_panel_disable() in their shutdown() function,
whan the panel is already disabled and then shutdown() is called.

Reading the backlight code it seems safe to call
the backlight_(enable|disable) several times.

v3:
- Improve comments, fix grammar (Laurent)
- Do not fail in drm_panel_of_backlight() if no DT support (Laurent)
- Log if backlight_(enable|disable) fails (Laurent)
- Improve drm_panel_of_backlight() docs
- Updated changelog with backlight analysis (triggered by Laurent)

v2:
- Drop test of CONFIG_DRM_PANEL in header-file (Laurent)
- do not enable backlight if ->enable() returns an error

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Cc: Sean Paul <sean@poorly.run>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20191207140353.23967-3-sam@ravnborg.org