drm/amdgpu: change pcie_gen_cap magic code to macro

This patch changes pcie_gen_cap magic code to macro to make it more
readable.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Huang Rui <ray.huang@amd.com>
Cc: Eric Huang <JinHuiEric.Huang@amd.com>
Cc: Ken Wang <Qingqing.Wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>