drm/vc4: plane: Change ptr0_offset to an array

The BCM2712 will have a fairly different dlist, that will feature one
Pointer 0 word for each plane.

Let's prepare by changing the ptr0_offset variable that holds the offset
in a dlist of the pointer 0 word to an array.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>