drm/amd/display: Use real BE and FE index to program regs.

In case of some pipes are fused, pipe_idx should not
be used to program pipe regs. Instead of that, BE and FE
inst number should be used for reg index.

Signed-off-by: Yongqiang Sun <yongqiang.sun@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>