drm/amdgpu: do some register access cleanup in nbio v7_9

Use WREG_SOC15x() instead of WREG32(SOC15_REG_OFFSET())

Signed-off-by: Le Ma <le.ma@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>