drm/amdgpu: Remove ATC L2 access for MMHUB 2.1.x

MMHUB 2.1.x versions don't have ATCL2. Remove accesses to ATCL2 registers.

Since they are non-existing registers, read access will cause a
'Completer Abort' and gets reported when AER is enabled with the below patch.
Tagging with the patch so that this is backported along with it.

v2: squash in uninitialized warning fix (Nathan Chancellor)

Fixes: 8795e182 ("PCI/portdrv: Don't disable AER reporting in get_port_device_capability()")

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Guchun Chen <guchun.chen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org