to_user_ptr() simply casts a pointer passed as u64 from user space
to void __user * correctly. Using this lets us get rid of all the
tiresome casts.

The idea came from Chris Wilson <chris@chris-wilson.co.uk>.
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>