video: stm32: ltdc: properly search the first available panel

video: stm32: ltdc: properly search the first available panel

Initially there was only one DSI bridge with one panel attached to this
device. This explained the call to uclass_first_device_err(UCLASS_PANEL,
...) which worked fine at the time.

Now that multiple bridges and panels, with different technologies, can
be plugged onto the board this way to get the panel device is outdated.

The lookup is done is two steps. First we circle through the
UCLASS_VIDEO_BRIDGE, and once we get one, we search through its
endpoints until we get a UCLASS_PANEL device available.

Acked-by: Yannick Fertre <yannick.fertre@foss.st.com>
Signed-off-by: Raphael Gallais-Pou <raphael.gallais-pou@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>