c/main: Follow style of using vk variable for vk_bundle interactions

src/xrt/compositor/main/comp_compositor.c

src/xrt/compositor/main/comp_layer.c

src/xrt/compositor/main/comp_renderer.c

src/xrt/compositor/main/comp_swapchain.c

src/xrt/compositor/main/comp_target_swapchain.c

src/xrt/compositor/main/comp_window_direct_nvidia.c

src/xrt/compositor/main/comp_window_vk_display.c