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