drm/amdgpu: don't use ATRM for external devices

The ATRM ACPI method is for fetching the dGPU vbios rom
image on laptops and all-in-one systems.  It should not be
used for external add in cards.  If the dGPU is thunderbolt
connected, don't try ATRM.

v2: pci_is_thunderbolt_attached only works for Intel.  Use
    pdev->external_facing instead.
v3: dev_is_removable() seems to be what we want

Link: drm/amd#2925


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