radv: Use an array to store descriptor sets.

radv: Use an array to store descriptor sets.

The vram_list linked list resulted in lots of pointer chasing.
Replacing this with an array instead improves descriptor set
allocation CPU usage by 3x at least (when also considering the free),
because it had to iterate through 300-400 sets on average.

Not a huge improvement as the pre-improvement CPU usage was only
about 2.3% in the busiest thread.

Reviewed-by: Dave Airlie <airlied@redhat.com>