drm/i915/gem: Safely acquire the ctx->vm when copying

As we read the ctx->vm unlocked before cloning/exporting, we should
validate our reference is correct before returning it. We already do for
clone_vm() but were not so strict around get_ppgtt().

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191106091312.12921-1-chris@chris-wilson.co.uk