drm/amdgpu: fix nbif 6.3.1 l1 low power not functional

drm/amdgpu: fix nbif 6.3.1 l1 low power not functional

The PCIe L1 low‑power settings for NBIF 6.3.1 were never applied due to
unresolved register mapping, which caused the relevant code to be compiled out.
As a result, the PCIe link could not enter L1/L23 power‑down states or transition to L0s.

Properly configure the link control register to enable L1 and L23 power‑down,
and permit L0s link transitions. Keep LTR disabled and let the PCI core enable it
only after verifying end‑to‑end root complex support across switches.

Fixes: 894c6d3522d1 ("drm/amdgpu: Add nbif v6_3_1 ip block support")
Signed-off-by: Yang Wang <kevinyang.wang@amd.com>
Signed-off-by: Kenneth Feng <Kenneth.feng@amd.com>
Reviewed-by: Kenneth Feng <kenneth.feng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit c2417f9fd7049d5a8d87eefd82fd6e36ba1ff7b6)
Cc: stable@vger.kernel.org