drm/amdkfd: report the real PCI bus number

Since the PCI bus number retrieved by PCI_BUS_NUM(pdev->devfn)
is wrong.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>