drm/amdgpu: drop legacy IO bar support

It was leftover from radeon where it was required for some
specific old hardware.  It hasn't been required for ages
and the driver already falls back to MMIO when legacy IO
is not available.  Legacy IO also seems to be problematic on
on some thunderbolt devices.  Drop it.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Nicholas Johnson <nicholas.johnson-opensource@outlook.com.au>