dm: video: Add a temporary work-around for old stdout var

dm: video: Add a temporary work-around for old stdout var

Boards with a saved environment may use 'lcd' in their stdout environment
variable, expecting that this will enable output to the LCD. When the board
moves to use driver model for video, this will no-longer work. Add a
work-around to fix this. A warning messages is printed, and we will remove
the work-around at the end of 2016.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Anatolij Gustschin <agust@denx.de>
Tested-by: Stephen Warren <swarren@nvidia.com>