divide the output pf s3c2410fb_calc_pixclk() by 2 which leaves us with a
rounding error if the result is odd.

Change to using DIV_ROUND_UP() to ensure that we always choose a higher
divisor and thus a lower frequency.
Signed-off-by: Ben Dooks <ben@simtec.co.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>