drm/i915/gt: Move scratch page into system memory on all platforms

The scratch page should never be accessed, and is only assigned as a
filler page to redirection invalid userspace access. It is not of a
performance concern and so we prefer to have a single consistent
configuration across all platforms, reducing the pressure on device
memory and avoiding the direct device access that would be required to
initialise the scratch page.

Signed-off-by: Chris Wilson <chris.p.wilson@intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220926155018.109678-1-matthew.auld@intel.com