This cl marks VK_KHR_xcb_surface as unsupported if we haven't
successfully loaded XCB. Also, an assert was added to make sure
vkCreateXcbSurfaceKHR is called with a valid connection.

Bug: b/139491466
Change-Id: Id4ed46102cd5c2535e38bf65dedbeea1cd252e6e
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/53229Tested-by: Alexis Hétu <sugoi@google.com>
Commit-Queue: Alexis Hétu <sugoi@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>