xf86drm: flex platform specifics into drmParsePciBusInfo

This will allow one to reuse the core drmGetDevices implementation on
other platforms. Keeping all the platform specifics in ParseFoo.

On the plus side this saves a bit of code :)

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>