drm/i915/gvt: Fix drm_format_mod value for vGPU plane

Physical plane's tiling mode value is given directly as
drm_format_mod for plane query, which is not correct fourcc
code. Fix it by using correct intel tiling fourcc mod definition.

Current qemu seems also doesn't correctly utilize drm_format_mod
for plane object setting. Anyway this is required to fix the usage.

v3: use DRM_FORMAT_MOD_LINEAR, fix comment

v2: Fix missed old 'tiled' use for stride calculation

Fixes: e546e281 ("drm/i915/gvt: Dmabuf support for GVT-g")
Cc: Tina Zhang <tina.zhang@intel.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Colin Xu <Colin.Xu@intel.com>
Reviewed-by: Colin Xu <Colin.Xu@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>