drm/amd/powerplay: use REG32_PCIE wrapper instead for sw smu

This patch uses REG32_PCIE wrapper instead of writting pci_index2 and reading
pci_data2. This sequence should be protected by pcie_idx_lock.

Signed-off-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>