radv: introduce perf test env var and allow to enable chaining

radv: introduce perf test env var and allow to enable chaining

We have some features that seem to slow things down or cause other
possible undesireable side effects, but it would be nice to test
games etc with them easily.

I forsee multisample DCC and maybe some shader opt changes using this.

For now use it for batch chaining.

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Signed-off-by: Dave Airlie <airlied@redhat.com>