Factor out common code to amdgpu_probe()

Keep the distinct pci/platform screen management in the separate probe
entry point and fold the rest into a single function.

v2: Rebase

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
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