graphics/mesa-devel: depend on mesa-dri for EGL/Vulkan ICD

Some users get confused and try to remove mesa-dri only to break Vulkan.
However, removing mesa-dri is only supported when mesa-devel is built
in standalone mode (DEFAULT_VERSIONS+=gl=mesa-devel).

$ pkg delete -f mesa-dri
$ vkcube-xcb
vkEnumerateInstanceExtensionProperties failed to find the VK_KHR_surface extension.

Do you have a compatible Vulkan installable client driver (ICD) installed?
Please look at the Getting Started guide for additional information.