drm: use DIV_ROUND_UP helper macro for calculations

Replace open coded divisor calculations with the DIV_ROUND_UP kernel
macro for better readability.
Issue found using coccinelle:
@@
expression n,d;
@@
(
- ((n + d - 1) / d)
+ DIV_ROUND_UP(n,d)
|
- ((n + (d - 1)) / d)
+ DIV_ROUND_UP(n,d)
)

Signed-off-by: Wambui Karuga <wambui.karugax@gmail.com>
Acked-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20191025094907.3582-1-wambui.karugax@gmail.com