drm: Implement drm_get_pci_dev() dummy for !PCI

Implementing a dummy of this function allows drivers that use it to be
built on platforms that don't have PCI. This can happen for example if
the nouveau driver is built on Tegra without PCI enabled (or on 64-bit
ARM where PCI is not yet implemented).

Signed-off-by: Thierry Reding <treding@nvidia.com>