tdfxfb: add mode_option module parameter

Small step toward unification of mode setting parameter.  This is required
to fix the Bugzilla's bug 9847

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>