drm/i915: Encapsulate lmem rpm stuff in intel_runtime_pm

Runtime pm is not really per GT, therefore it make sense to
move lmem_userfault_list, lmem_userfault_lock and
userfault_wakeref from intel_gt to intel_runtime_pm structure,
which is embedded to i915.

No functional change.

v2:
- Fixes the code comment nit. [Matt Auld]

Signed-off-by: Anshuman Gupta <anshuman.gupta@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221027092242.1476080-2-anshuman.gupta@intel.com