cirrusfb: do not calculate line length twice

A line length is calculated twice: first in the cirrusfb_decode_var() then
in the cirrusfb_set_par_foo().

Use the first calculated value.  A nice side effect is that 32bpp mode
works now.

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Cc: Geert Uytterhoeven <geert.uytterhoeven@gmail.com>
Cc: Arthur Marsh <arthur.marsh@internode.on.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>