sm501: restructure init to allow only 1 fb on an SM501

Add the ability to register only one of the two possible main framebuffer
devices on the SM501 by passing platform data for only the framebuffer
that you are interested in having.

As a side note, we update the init sequence to commonise the code that is
executed twice, and fix a pair of missing frees that we didn't do on
framebuffer exit, such as freeing the fb's cmap.

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>