drm/amd/display: Fix HSplit causing increase in DSC Slice Count

[Why?]
HSplit should not affect DSC slice count. Can cause improper timings to
be applied for certain modes.

[How?]
No longer change DSC Slice count based on HSplit.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Dillon Varone <dillon.varone@amd.com>
Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>