drm/i915/userptr: Fix error handling of mutex_lock_killable()

mutex_lock_killable() returns -EINTR on failure, not the anticipate bool
return like trylock. (Oh no, not again.)

Fixes: 484d9a84

 ("drm/i915/userptr: Avoid struct_mutex recursion for mmu_invalidate_range_start")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190115221118.13304-1-chris@chris-wilson.co.uk


Reviewed-by: Matthew Auld <matthew.auld@intel.com>