radv: detect command buffers that do no work and drop them (v2)

radv: detect command buffers that do no work and drop them (v2)

If a buffer is just full of flushes we flush things on command
buffer submission, so don't bother submitting these.

This will reduce some CPU overhead on dota2, which submits a fair
few command streams that don't end up drawing anything.

v2: reorganise loop to count first then malloc,
rename some vars (Bas)

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