xfree86: Fix potentially NULL reference to platform device's PCI device

xf86_platform_devices[i].pdev may be NULL in cases we fail to parse the
busid in config_udev_odev_setup_attribs() (see also [1], [2]) such as
when udev does not give use ID_PATH. This in turn leads to
platform_find_pci_info() being not called and pdev being NULL.

[1]: xorg/xserver#993
[2]: xorg/xserver#1076

Reviewed-by: Zoltán Böszörményi <zboszor@gmail.com>
Signed-off-by: Povilas Kanapickas <povilas@radix.lt>
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