drm/i915/gvt: Fix tiled memory decoding bug on BDW

Commit b244ffa1 ("drm/i915/gvt: Fix drm_format_mod value for vGPU
plane") introduced a regression issue to the tiled memory decoding on BDW.

This patch can fix this issue.

Here is the issue detail: https://github.com/intel/gvt-linux/issues/61



v1->v2:
- Refine the commit message. (Zhenyu)

Fixes: b244ffa1("drm/i915/gvt: Fix drm_format_mod value for vGPU plane")
Signed-off-by: Tina Zhang <tina.zhang@intel.com>
Cc: stable@vger.kernel.org # v4.19+
Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>