Implement Linux simple display mode with vulkan backend
through VK_KHR_display.

Added value 'EGL_PLATFORM_VULKAN_DISPLAY_MODE_SIMPLE_ANGLE' for
attribute EGL_PLATFORM_ANGLE_NATIVE_PLATFORM_TYPE_ANGLE to identify
the new simple display mode. Also reserved
EGL_PLATFORM_VULKAN_DISPLAY_MODE_HEADLESS_ANGLE for headless mode.

How to enable:
Add

```
use_x11=false
angle_vulkan_display_mode="simple" # default value
```

into args.gn, then compile with linux vulkan args.

Bug: angleproject:5214
Change-Id: I1247585b9de8b55df106aba99322281f1c183203
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2494320
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>