GEM: Wait for existing rendering to complete before writing relocation data.

GEM: Wait for existing rendering to complete before writing relocation data.

This should already have been generally safe since we don't change contents
and put in new relocations between execbufs, so if we were writing in a new
relocation then we'd already waited rendering to complete when we moved
the target of the relocation.  However, doing the right thing will be required
if we do buffer reuse.