drm/i915/ttm: fix CCS handling

In the Linux kernel, the following vulnerability has been resolved:

drm/i915/ttm: fix CCS handling

Crucible + recent Mesa seems to sometimes hit:

GEM_BUG_ON(num_ccs_blks > NUM_CCS_BLKS_PER_XFER)

And it looks like we can also trigger this with gem_lmem_swapping, if we
modify the test to use slightly larger object sizes.

Looking closer it looks like we have the following issues in
migrate_copy():

- We are using plain integer in various places, which we can easily
overflow with a large object.

- We pass the entire object size (when the src is lmem) into
emit_pte() and then try to copy it, which doesn't work, since we
only have a few fixed sized windows in which to map the pages and
perform the copy. With an object > 8M we therefore aren't properly
copying the pages. And then with an object > 64M we trigger the
GEM_BUG_ON(num_ccs_blks > NUM_CCS_BLKS_PER_XFER).

So it looks like our copy handling for any object > 8M (which is our
CHUNK_SZ) is currently broken on DG2.

Testcase: igt@gem_lmem_swapping
(cherry picked from commit 8676145eb2f53a9940ff70910caf0125bd8a4bc2)