st/va: add support to export a surface as dmabuf

st/va: add support to export a surface as dmabuf

I.e. implements:
VaAcquireBufferHandle
VaReleaseBufferHandle
for memory of type VA_SURFACE_ATTRIB_MEM_TYPE_DRM_PRIME

And apply relatives change to:
vlVaMapBuffer
vlVaUnMapBuffer
vlVaDestroyBuffer

Implementation inspired from cgit.freedesktop.org/vaapi/intel-driver

Tested with gstreamer-vaapi with nouveau driver.

Signed-off-by: Julien Isorce <j.isorce@samsung.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Christian König <christian.koenig@amd.com>