drm/amd/display: Remove useless assignments

[WHAT & HOW]
"split_pipe" are assigned to test_pipe and then immediately are updated
to other values. The same also applies to "status" as well.

Similarly, "id", "dwb" and "unused_dpps" are assigned but the functions
immediately return, and thus they have no effects.

As a results, the assignments removed.

This fixes 5 UNUSED_VALUE issues reported by Coverity.

Reviewed-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Roman Li <roman.li@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>