va: Install library and generate its GI

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Currently libgstva is only exposed internally to gst-plugins-bad. No headers are installed, no documentation generated, neither gobject-introspection files.

This patch turn on all if we decide to offer the library to users of gst-plugins-bad.

Documentation generation is dropped because the plugin is not currently build by CI.

As in the case of d3d11, no API stability promises are made.

Cc: @He_Junyan, @seungha.yang, @ndufresne

Edited by Víctor Manuel Jáquez Leal

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