drm/vmwgfx: Fix surface framebuffer check for guest-backed surfaces

With guest-backed surfaces, surface->sizes == NULL, causing a kernel oops.
Use the base_size member instead.

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