radv: Reinitialise loaderMagic when allocating a cached command buffer

radv: Reinitialise loaderMagic when allocating a cached command buffer

This must be set to ICD_LOADER_MAGIC by vkAllocateCommandBuffers, which
was being done when allocating a new buffer but not when reusing an
existing one in the cache. This would hit an assertion and crash in
debug builds of the Vulkan loader.

Fixes: 682248db451f ("radv: Cache command buffers in command pool.")
Signed-off-by: Alex Smith <asmith@feralinteractive.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>