We shouldn't allow write for exposed guest display buffer which
doesn't make sense. So explicitly set read only flag for display
dmabuf allocated object.

Fixes: e546e281 ("drm/i915/gvt: Dmabuf support for GVT-g")
Cc: Tina Zhang <tina.zhang@intel.com>
Acked-by: Tina Zhang <tina.zhang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20191212084614.1100-1-zhenyuw@linux.intel.com