drm/radeon: change rdev->ddev to rdev_to_drm(rdev)

This patch changes the way "drm_device" is accessed. It uses "rdev_to_drm(rdev)"
instead of accessing the struct member directly.

Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Tested-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Wu Hoi Pok <wuhoipok@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>