drm/vmwgfx: Tighten the security around buffer maps

Make sure only buffer objects that are referenced by the client can be mapped.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>

diff --cc drivers/gpu/drm/vmwgfx/vmwgfx_resource.c
Simple merge