xf86drm: fix subsystem type lookup for virtio mmio-based devices

Currently the code assumes that a virtio based device is always located
on the PCI bus.

Modify the parser to make it check the device's parent directory to
determine on which bus it is located.

Output for virtio-pci is the PCI bus.
Output for virtio-mmio is the Platform bus.

Signed-off-by: Vasyl Vavrychuk <vasyl.vavrychuk@opensynergy.com>
Signed-off-by: Mikhail Golubev <Mikhail.Golubev@opensynergy.com>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>