WIP: panfrost: Make image plane layout initialization gallium agnostic
We will need pretty much the same layout initialization for Vulkan, so let's make the panfrost_setup_slices() implementation gallium-agnostic.
While at it, we get rid of the slice states that were mixed with the layout description (initialized and checksum_bo) so we have an object that only describes the layout, nothing more.
/cc @alyssa