drm/amdgpu: Retire pcie_gen3_enable function

Not needed since from vi. drop the function so
we don't duplicate code when introduce new asics.

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Likun Gao <Likun.Gao@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>