drm/amd: Add dedicated helper for amdgpu_device_find_parent()

drm/amd: Add dedicated helper for amdgpu_device_find_parent()

There are a few cases that code walks up the topology to find the
link partner of the integrated switch in a dGPU.  Split this out
to a helper and call in all places.

This does have a functional change that amdgpu_device_gpu_bandwidth()
doesn't cache the internal link but only the parent.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>