drm/radeon: check if device is root before getting pci speed caps

Check if the device is root rather before attempting to see what
speeds the pcie port supports.  Fixes a crash with pci passthrough
in a VM.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=109366


Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>