drm/xe/ggtt: fix GGTT scratch usage for DG2

Scratch page is in VRAM, and therefore requires 64K GTT layout. In GGTT
world this just means having 16 consecutive entries, with 64K GTT
alignment for the GTT address of the first entry (also matching physical
alignment). However to keep things simple just dump it into system
memory, like we already do for ppGTT.  While we are here, also give it
known default value.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>