drm/vmwgfx: Reorganize the fence wait loop

Reorganize the fence wait loop somewhat to make it look more like the
examples in set_current_state() kerneldoc, and add some code comments.

Also if we're about to time out, make sure we check again whether the fence
is actually signaled.

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