board: cm_fx6: Enable DM support for video, fix build error

board: cm_fx6: Enable DM support for video, fix build error

Enable driver model for Video to remove the following
compile warning on CM-FX6 SOM based target:

===================== WARNING ======================
This board does not use CONFIG_DM_VIDEO Please update
the board to use CONFIG_DM_VIDEO before the v2019.07 release.
====================================================

This change introduced build error as shown:

LD      u-boot
drivers/built-in.o: In function ipu_displays_init'
arm-linux-ld.bfd: BFD (GNU Binutils) 2.29.1 assertion fail elf32-arm.c:9509
Makefile:1621: recipe for target 'u-boot' failed
make: *** [u-boot] Error 1

The DM converted video driver calls ipu_displays_init
in its probe, which inturn calls relevant board_video_skip.
Defining ipu_displays_init in the board file fixes build error.
Target was compile tested, build was clean.

Signed-off-by: Suniel Mahesh <sunil.m@techveda.org>