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

drivers/gpu/drm/i915/gt/intel_ggtt_fencing.c

@@ -233,10 +233,12 @@ static int fence_update(struct i915_fence_reg *fence,
 	int ret;
 
 	if (vma) {
-		GEM_BUG_ON(!i915_vma_is_map_and_fenceable(vma));
 		GEM_BUG_ON(!i915_gem_object_get_stride(vma->obj) ||
 			   !i915_gem_object_get_tiling(vma->obj));
 
+		if (!i915_vma_is_map_and_fenceable(vma))
+			return -EINVAL;
+
 		ret = i915_vma_sync(vma);
 		if (ret)
 			return ret;