drm/amdgpu: Remove leftover igp_lane_info

Variable igp_lane_info always is 0. 0 & any value = 0 and false.
In this way, all сonditional statements will false.

The code was leftover from when the code was ported from radeon
where igp_lane_info was derived from the vbios on supported
platforms.

[update commit message - Alex]

Signed-off-by: Grigory Vasilyev <h0tc0d3@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>