drm/i915: Use RCU for unlocked vm_idr lookup

Since i915_address_space is now RCU protected, we can do the vm_idr lookup
without taking the vm_idr_mutex, just with the rcu_read_lock() instead.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190830180325.7755-1-chris@chris-wilson.co.uk