If a device extension isn't enabled, then the associated extension
functions shouldn't be exposed through vkGetDeviceProcAddr().

Bug b/117974925

Change-Id: I828cf9ff9d8aaf22c15a5379cfb9d93612d23360
Tests: dEQP-VK.api.version_check.entry_points
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/31696
Presubmit-Ready: Alexis Hétu <sugoi@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>