libavcodec/amfenc: Vulkan initialization support for encoder.

libavcodec/amfenc: Vulkan initialization support for encoder.

Added linux support for amf encoder through vulkan.

To use h.264(AMD VCE) encoder on linux amdgru-pro version 19.20+ and
amf-amdgpu-pro package(amdgru-pro contains, but does not install
automatically) are required.

This driver can be installed using amdgpu-pro-install script in
official amd driver archive.

Initialization of amf encoder occurs in this order:
1) trying to initialize through dx11(only windows)
2) trying to initialize through dx9(only windows)
3) trying to initialize through vulkan

Only Vulkan initialization available on linux.