fbdev: add removed option "mode" to keep compatibility

Add removed option "mode" to keep compatibility with existing setups.  The
option is back for pm2fb, tridentfb and vt8623fb drivers.

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Cc: "Antonino A. Daplas" <adaplas@pol.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>