We need to iterate over all pixel pipelines to get overall value.

Changes from v4 -> v5:
- switch back to pixel pipe 0 to prevent GPU hang
- PIXELS_RENDERED_2D is exposed for 2D pipe
Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>