vdpau: Report DRI2 VDPAU driver name as 'sunxi' on Allwinner hardware

Try to load the 'sunxi_cedar_mod' kernel module. And if it loads
successfully, then report the DRI2 VDPAU name as 'sunxi'. This allows to
use libvdpau-sunxi without setting the VDPAU_DRIVER environment variable.

Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>