drm: xlnx: zynqmp: Add zynqmp_disp_layer_is_video() to simplify the code

Add a new function zynqmp_disp_layer_is_video() to simplify the code
that judges if a layer is the video layer.

Acked-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Quanyang Wang <quanyang.wang@windriver.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
[Renamed is_layer_vid() to zynqmp_disp_layer_is_video()]]
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>