atmel_lcdfb: support native-mode display-timings

atmel_lcdfb: support native-mode display-timings

When a device tree set a display-timing using native-mode
then according to the bindings doc this should:

    native-mode:
    The native mode for the display, in case multiple
    modes are provided.
    When omitted, assume the first node is the native.

The atmel_lcdfb used the last timing subnode and did not
respect the timing mode specified with native-mode.

Introduce use of of_get_videomode() which allowed
a nice simplification of the code while also
added support for native-mode.

As a nice side-effect this fixes a memory leak where the
data used for timings and the display_np was not freed.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Nicolas Ferre <nicolas.ferre@microchip.com>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>