vulkan: move magma connection state to util_sync_provider_magma

This removes magma connection state from vk_device and puts it
in a magma-specific container similar to util_sync_provider_drm.

Also remove an unused local.

Bug:501928877
Change-Id: I146e78fdf1b4cae4e5d5da8eac8f181ae7b225fd
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/mesa/+/1571705
Reviewed-by: David Gilhooley <dgilhooley@google.com>
Commit-Queue: Craig Stout <cstout@google.com>