radv: overhaul fragment shader sample positions.

radv: overhaul fragment shader sample positions.

The current code was broken, and I decided to redesign it instead.

This puts the sample positions for all samples into the queue
constant descriptor buffer after all the spill/ring descriptors.

It then uses a single offset register to point how far into the
samples the samples for num_samples are. This saves one user sgpr
and means we only generate the sample position data in the rare
single case where we need it currently.

This doesn't fix the failing CTS tests without the followup
fix.

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