drm/i915: use xa_lock/unlock for fpriv->vm_xa lookups

We don't need the absolute speed of rcu for this. And
i915_address_space in general dont need rcu protection anywhere else,
after we've made gem contexts and engines a lot more immutable.

Note that this semantically reverts

commit aabbe344
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Fri Aug 30 19:03:25 2019 +0100

    drm/i915: Use RCU for unlocked vm_idr lookup

except we have the conversion from idr to xarray in between.

v2: kref_get_unless_zero is no longer required (Maarten)

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jon Bloomfield <jon.bloomfield@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: "Thomas Hellström" <thomas.hellstrom@linux.intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20210902142057.929669-10-daniel.vetter@ffwll.ch