drm/i915/gt: Restore check for invalid vma for fencing

Apparently we do try and attach a fence to an invalid vma (during
execbuf) so we cannot simply assert it never happens and report EINVAL
instead.

Fixes: dec9cf9e

 ("drm/i915/gt: Pull restoration of GGTT fences underneath the GT")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200316205450.15843-1-chris@chris-wilson.co.uk