video: fsl_dcu_fb: refactor init functions

Move dcu-related code to fsl_dcu_probe_common, keep in video_hw_init()
only legacy video stack (filling GraphicPanel struct etc.).

Add wrappers for all init functions, that will let to provide
struct fb_info as an additional param (needed for further moving it from
the global scope to driver private data struct in DM converted driver).

Signed-off-by: Igor Opaniuk <igor.opaniuk@toradex.com>
Reviewed-by: Oleksandr Suvorov <oleksandr.suvorov@toradex.com>