drm/amdgpu: consolidate the access helpers in nbio v7_9

Use WREG32_SOC15_EXT to write registers with address larger than 32bit.

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>