layers: DS: populate features correctly
layers: DS: populate features correctly

- Remove the redundant first fetching of the device features. This was
  always clobbered.
- Treat pEnabledFeatures==NULL as disabling all features, as per
  30.1 of the spec.

Signed-off-by: Chris Forbes <c3279c50043e18e9442c1ae071cafa0e7a4ca967@google.com>

KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools