drm/i915: Add ww locking to vm_fault_gtt

We want to start requiring the reservation_lock instead of obj->mm.lock
for pinning objects, take the ww lock inside vm_fault_gtt as a first step
towards the legacy lock removal.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200819140904.1708856-23-maarten.lankhorst@linux.intel.com


Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>