drm: Use u64_to_user_ptr() helper for blob ioctls

Remove the ugly sparse casts by using the helper u64_to_user_ptr()
instead.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161127170910.29106-1-chris@chris-wilson.co.uk