drm/radeon: Use pci_dev_id() to simplify the code

PCI core API pci_dev_id() can be used to get the BDF number for a pci
device. We don't need to compose it mannually. Use pci_dev_id() to
simplify the code a little bit.

Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>