drm/i915/gtt: stop caching the scratch page

Normal users shouldn't be hitting this, likely this would indicate a
userspace bug. So don't bother caching, which should be safe now that we
manually flush the page.

Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211028092638.3142258-2-matthew.auld@intel.com