ARM: i.MX video: declare displays and display_count publicly

Declare displays[] and display_count in imx-common/video.h to
prevent "Should it be static?" messages when compiling board
files with "make C=1".

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>
Merge branch 'staging' of github.com:boundarydevices/u-boot-imx6 into staging