drm/i915/gvt: set guest display buffer as readonly

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