drm/i915: Refactor userfault_wakeref to re-use

Refactor userfault_wakeref to re-use for discrete lmem mmap mapping
as well, as on discrete GTT mmap are not supported. Moving
userfault_wakeref from ggtt to gt structure.

Signed-off-by: Anshuman Gupta <anshuman.gupta@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220913152714.16541-2-anshuman.gupta@intel.com