iris: Reorganise execbuf to have a single point of failure

iris: Reorganise execbuf to have a single point of failure

Propagate the failure from GEM_EXECBUFFER2, cleanup then report failure
if need be. We retain the current behaviour to abort() at the first sign
of trouble -- for a non-robustness context, arguably this is the right
thing to do as the client cannot recover, and the system state is lost.
How to properly integrate with KHR_robustness and reset-strategy is
left as a future exercise.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>