drm/amdgpu: add param to specify fw bo location for front-door loading

This param can help isolating data path issues on new systems in early phase.

Signed-off-by: Le Ma <le.ma@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>