dm: pci: Add a dm_ prefix to pci_bus_find_bdf()

Most driver model PCI functions have a dm_ prefix. At some point, when the
old code is converted to driver model and the old functions are removed, we
will drop that prefix.

For consistency, we should use the dm_ prefix for all driver model
functions. Update pci_bus_find_bdf() accordingly.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>