drm/amdgpu: remove structurally dead code for amd_gmc

This code cannot be reached: return sysfs_emit(buf, "UNK....)

Signed-off-by: Jesse Zhang <Jesse.Zhang@amd.com>
Reviewed-by: Tim Huang <Tim.Huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>