This cl marks VK_KHR_xlib_surface as unsupported if we haven't
successfully loaded X11. Also, an assert was added to make sure
vkCreateXlibSurfaceKHR is called with a valid display.

Bug: b/139491466
Change-Id: I6da3755a15e09320e5715b2036043acbb31c0c35
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/53168Tested-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>