drm/rockchip: use DIV_ROUND_UP macro for calculations.

Replace the open coded calculation with the more concise and readable
DIV_ROUND_UP macro.

Signed-off-by: Wambui Karuga <wambui.karugax@gmail.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20200109142057.10744-1-wambui.karugax@gmail.com