drm: xlnx: zynqmp_dpsub: Add support for live video input

Add partial live video support, with a single video input that bypasses
blending. Skip registration of the DRM device in that case, but register
the DRM bridge instead. The DRM device will be created by the driver for
the display controller in the PL.

Full live video mode with concurrent usage of the video and gfx inputs,
and blending in the DPSUB video pipeline, is currently unsupported.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>