Ensure that the default display parameter passed in via the
device's platform data is valid. It turns out when mach-bast.c
was updated, the default_display was set outside of the display
array bounds, causing a panic on startup.

If the default_display is bigger than num_displays, then generate
an error and refuse to initialise the driver.
Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Cc: "Antonino A. Daplas" <adaplas@pol.net>
Cc: Vincent Sanders <vince@simtec.co.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>