drm/amdgpu: don't use pci_is_thunderbolt_attached()

It's only valid on Intel systems with the Intel VSEC.
Use dev_is_removable() instead.  This should do the right
thing regardless of the platform.

Link: https://gitlab.freedesktop.org/drm/amd/-/issues/2925


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