drm/amdgpu: clean up vbios fetching code

After splitting the logic between APU and dGPU,
clean up some of the APU and dGPU specific logic
that no longer applied.

Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>