video: Provide an API to access video parameters

Create a basic API to provide access to video parameters such as screen
size, and to position the cursor on the screen. Also add a prototype
for video_display_bitmap() which was missing.

Signed-off-by: Stefan Reinauer <reinauer@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Anatolij Gustschin <agust@denx.de>