drm/vc4: Define missing PITCH0_SINK_PIX field

This is needed to support X/Y negative placement of planes using
T-format buffers.

Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180803092231.26446-2-boris.brezillon@bootlin.com