drm/radeon/kms: add missing copy from user

This hasn't mattered up until the ioctl started using the value, and it fell
apart.

fixes fd.o 29340, Ubuntu LP 606081

[airlied: cleaned up whitespace and don't need an error before pushing]

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>