drm/i915: Add object locking to vm_fault_cpu

Take a simple lock so we hold ww around (un)pin_pages as needed.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210323155059.628690-24-maarten.lankhorst@linux.intel.com