radv: fix setting the number of rectangles when it's dyanmic

We need to know the number of rectangles.

This fixes new CTS dEQP-VK.draw.discard_rectangles.dynamic_*.

Fixes: 5db0bf99 ("radv: Implement VK_EXT_discard_rectangles.")
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
(cherry picked from commit 53dfff1c)