Add functions to register various video devices like venc, osd,
vpbe and the v4l2 display driver for dm644x.

Change dm644x_init_video() to make room for display related
configuration. Register the vpfe or vpbe devices only if valid
display configuration is provided to make sure boards without
video support dont erroneously register video devices.
Signed-off-by: Manjunath Hadli <manjunath.hadli@ti.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>