Double buffering support

'psplash' uses only one buffer which can cause tearing artifacts. This
change uses the FBIOPAN_DISPLAY ioctl as a way to handle page flipping
and also does that after a vsync interrupt to remove any tearing issues.

Signed-off-by: Andrei Gherzan <agherzan@hanoverdisplays.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>