va: fix libgudev in va plugin and make it optional

va: Make libgudev dependency optional.

libgudev is a problematic dependency, particularly in sandboxed
environments, such as flatpak.

This patch implements a way to get the available VA devices using
brute-forced traverse of /dev/drm/renderD* directory. Thus usable in
those sandboxed environments.

va: meson: Move back libgudev dependency to plugin.

When move the libgstva, libgudev dependency was moved as part of the
library, though it's not use by the library but the plugin. This patch
moves back libgudev dependency to the plugin.

Also HAVE_LIBDRM is move to the library which is the one who use it.
Edited by Víctor Manuel Jáquez Leal

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