drm/i915/gt: introduce vm->scratch_range callback

The callback will be responsible for setting scratch page PTEs for
specified range. In contrast to clear_range it cannot be optimized to nop.
It will be used by code adding guard pages.

Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Signed-off-by: Andrzej Hajda <andrzej.hajda@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230308-guard_error_capture-v6-1-1b5f31422563@intel.com