drm/radeon: use pcie_get_readrq() and pcie_set_readrq() to simplify code

Use pcie_get_readrq() and pcie_set_readrq() functions to simplify code.

Signed-off-by: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>