cirrusfb: check_var improvements

Break cirrusfb_decode_var() function into two parts:
cirrusfb_check_pixclock() which can be called from the
cirrusfb_check_var() aand merge rest into the cirrusfb_set_par_foo().
This allows rejecting modes with too high pixclock before before any
change to hardware state (and a console is messed up).

Also, fix RGB field's lengths for 8bpp modes to correct ones so X11 works
with fbdev driver with cirrusfb.

Kill some redundant function calls or register loads.

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>