cirrusfb: use modedb and add mode_option parameter

Use modedb for initial mode instead of table of few predefined modes.

Add mode_option module parameter as a step toward unification of frame
buffers' parameters.

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>