drm/xe/ppgtt: fix scratch page usage on DG2

On DG2 when running the xe_vm IGT, the kernel generates loads of CAT
errors and GT resets (sometimes at least).  On small-bar systems seems
to trigger a lot more easily (maybe due to difference in allocation
strategy). Appears to be related to scratch, since we seem to use the
64K TLB hint on scratch entries, even though the scratch page is a 4K
vram page. Bumping the scratch page size and physical alignment seems
to fix it. Or at least we no longer hit:

[  148.872683] xe 0000:03:00.0: [drm] Engine memory cat error: guc_id=0
[  148.872701] xe 0000:03:00.0: [drm] Engine memory cat error: guc_id=0
[  148.875108] WARNING: CPU: 0 PID: 953 at drivers/gpu/drm/xe/xe_guc_submit.c:797

However to keep things simple, so we don't have to deal with 64K TLB
hints, just move the scratch page into system memory on platforms that
require 64K VRAM pages.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>