radv: fix VK_EXT_memory_budget if one heap isn't available

When the visible VRAM size is equal to the VRAM size only two
heaps are exposed.

This fixes dEQP-VK.api.info.device.memory_budget.

Cc: 19.0 19.1 <mesa-stable@lists.freedesktop.org>
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-By: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
(cherry picked from commit d3781512)