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 <mripard@kernel.org>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240621152055.4180873-27-dave.stevenson@raspberrypi.com


Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>