dsp: don't use registered_fb[1]

dsp: don't use registered_fb[1]

According to dsp_fbexport(), the dsp code handles only the first
registered framebuffer, so using registered_fb[1] in mbox_fbctl_upd is
wrong. registered_fb[0] must be used instead.
This code has also the side effect to oops the kernel as
registered_fb[1] is null and omapfb_update_window_async doesn't handle
this case. The fix for omapfb_update_window_async is sent in an other
mail.

Signed-off-by: Arnaud Patard <arnaud.patard@rtp-net.org>
Acked-by: Hiroshi DOYU <Hiroshi.DOYU@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>