drm/i915: Optimize DIV_ROUND_CLOSEST call

DIV_ROUND_CLOSEST is faster if the compiler knows it will only be
dealing with unsigned dividends. This optimization rips 32 bytes of
binary code on x86_64.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: David Airlie <airlied@linux.ie>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>