drm/i915: Fix the sgt.pfn sanity check

For the device local-memory case, sgt.pfn will always be equal to zero,
since we instead use sgt.dma. Also, for device local-memory it is
perfectly valid for it to start from zero anyway, so no need to add a
new check for that either.
Signed-off-by: Kui Wen <kui.wen@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20210119133106.66294-1-matthew.auld@intel.com

drivers/gpu/drm/i915/i915_mm.c

@@ -62,7 +62,7 @@ static int remap_sg(pte_t *pte, unsigned long addr, void *data)
 {
 	struct remap_pfn *r = data;
 
-	if (GEM_WARN_ON(!r->sgt.pfn))
+	if (GEM_WARN_ON(!r->sgt.sgp))
 		return -EINVAL;
 
 	/* Special PTE are not associated with any struct page */