radv: zero the bo descriptor array when allocating a new set

this must be reset to avoid issues when using VK_DESCRIPTOR_BINDING_PARTIALLY_BOUND_BIT_EXT
when some descriptors in the set may not have been bound

fixes #4219

Fixes: 126d5adb11e ("radv: Use host memory pool for non-freeable descriptors.")

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8840>