1. Add support for Linux GPU info with vulkan backend: for
dev board without PCI support, try vulkan backend to get
GPU info.
2. Fix getVulkanInstance() issue when loading non-exist lib:
return NULL instance if open lib failed.

Bug: angleproject:5717
Change-Id: I03c11da25a8787496d098f6c9d6b4c53701383e4
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2734375Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>