drm/amdgpu: Change WREG32_RLC to WREG32_SOC15_RLC where inst != 0 (v2)

W/RREG32_RLC is hardedcoded to use instance 0. W/RREG32_SOC15_RLC
should be used instead when inst != 0.

v2: rebase

Signed-off-by: Victor Lu <victorchengchi.lu@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>