drm/vmwgfx: Fix an error path causing an oops.

An error happening before the snooper.image member had been set up
would cause a kfree of an arbitrary pointer. Set up the snooper.image
member early.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>