drm/i915: Guard error capture against unpinned vma

If we find an incompletely setup vma inside the request/engine at the
time of a hang, it may not have vma->pages initialised, so skip
capturing the object before we iterate over NULL.

Spotted by Matthew in preparation for using unpinned vma to track engine
state.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190110111522.11023-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gpu_error.c

@@ -1034,7 +1034,7 @@ i915_error_object_create(struct drm_i915_private *i915,
 	dma_addr_t dma;
 	int ret;
 
-	if (!vma)
+	if (!vma || !vma->pages)
 		return NULL;
 
 	num_pages = min_t(u64, vma->size, vma->obj->base.size) >> PAGE_SHIFT;