Copying 64 bit data from user space using get_user is not supported
on all architectures, and may result in the following build error.

ERROR: "__get_user_bad" [drivers/gpu/drm/drm.ko] undefined!

Avoid the problem by using copy_from_user.

Fixes: d34f20d6 ("drm: Atomic modeset ioctl")
Cc: Rob Clark <robdclark@gmail.com>
Signed-off-by: NGuenter Roeck <linux@roeck-us.net>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>