libva: Factor out base parts into an include file

To enable the VAAPI state tracker and drivers in mesa, it needs
libva.pc and the libva headers. To enable GLX support in libva,
it needs mesa to be compiled first. At the recipe level, this
would create a circular dependency between libva and mesa.

This is a preparation step before introducing a new libva recipe
variant to break the circular dependency.

Signed-off-by: Böszörményi Zoltán <zboszor@pr.hu>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>