arm64: dts: artik710_raptor: change HW overlay's priority

arm64: dts: artik710_raptor: change HW overlay's priority

Changed HW overlay's priority.

Video layer should have a lower priority than graphics layers to show
video and UI  images together on Display.
By doing so, Platform can pass video image on video layer, and UI
image on graphics layers that alpha channel on the region of video
layer to be shown is enabled.

Change-Id: I42e33f7188ca7935b6e622881a6fc1c3ff003ea4
Signed-off-by: Hoegeun Kwon <hoegeun.kwon@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>