video: consolidate splash screen alignment code

Code for checking "splashpos" environment variable is
duplicated in drivers, move it to the common function.
Call this function also in the bmp display command to
consider "splashpos" settings.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Acked-by: Otavio Salvador <otavio@ossystems.com.br>