drm/radeon: Use DIV_ROUND_UP()

Use DIV_ROUND_UP() instead of hand rolling it. Just a drive-by change.

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1479498793-31021-4-git-send-email-ville.syrjala@linux.intel.com