drm/amdgpu: assign fb_start/end in mmhub v9.4 interface

Align with mmhub v1.0.

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