vulkan/vkinfo: enable layers when -validate is specified

Enable layers when -validate is given, instead of unconditionally.  This
is a regression fix since vkinfo fails with

  vkCreateInstance failed: VK_ERROR_LAYER_NOT_PRESENT (-6)

since commit b1e7d59.

Change-Id: I42bfc474239896416e26ebf1b18cc65c9c045610
1 file changed