video: Do not show splash and U-Boot logo simultaneously

Currently, on imx6sabresd and gwventana boards, the company logo
and U-Boot logo are shown.

The correct behavior is to show only the company logo, if available,
and not both logos.

Reported-by: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Fabio Estevam <festevam@denx.de>
Tested-by: Tim Harvey <tharvey@gateworks.com> #gw_ventana