display: API for display driver

API for display drivers, supporting:
 * Turning on/off display blanking
 * Writing/Reading a bit map towards/from the display
 * Requesting framebuffer pointer
 * Setting display contrast and brightness
 * Querying display capabilities
 * Changing pixel format
 * Changing display orientation

Signed-off-by: Jan Van Winkel <jan.van_winkel@dxplore.eu>