drm/i915/gvt: invalidate old ggtt page when update ggtt entry

Previously only cancelled dma map of a ggtt page when the ggtt entry was
cleared. This patch will cancel dma map of an old ggtt page as well when
the ggtt entry is updated with new page address.

Fixes: 7598e870(drm/i915/gvt: Missed to cancel dma map for ggtt entries)
Signed-off-by: Hang Yuan <hang.yuan@linux.intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>