For speeding up rendering we need shadow framebuffers. This is currently
implemented in the gui functions. This does not work properly when two
users (splash and fbconsole) use the same framebuffer since in this case
two different shadow framebuffers will be used. This patch implements
shadowfb handling in the fb core directly. With this the fb device gets
a parameter 'shadowfb'. When this is true the fb core will allocate a
shadow fb and provide it to the users.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>