radv: Use correct buffer count with variable descriptor set sizes.

Fixes dEQP-VK.binding_model.descriptorset_random.sets16.noarray.ubolimitlow.sbolimitlow.imglimitlow.iublimitlow.frag.ialimitlow.0

CC: <mesa-stable@lists.freedesktop.org>
Closes: mesa/mesa#2607
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <mesa/mesa!4489>
24 jobs for !4489 with variable-buffer-count in 10 minutes and 1 second (queued for 4 seconds)
merge request